README-Thumb.txt
1 //===---------------------------------------------------------------------===// 2 // Random ideas for the ARM backend (Thumb specific). 3 //===---------------------------------------------------------------------===// 4 5 * Add support for compiling functions in both ARM and Thumb mode, then taking 6 the smallest. 7 8 * Add support for compiling individual basic blocks in thumb mode, when in a 9 larger ARM function. This can be used for presumed cold code, like paths 10 to abort (failure path of asserts), EH handling code, etc. 11 12 * Thumb doesn't have normal pre/post increment addressing modes, but you can 13 load/store 32-bit integers with pre/postinc by using load/store multiple 14 instrs with a single register. 15 16 * Make better use of high registers r8, r10, r11, r12 (ip). Some variants of add 17 and cmp instructions can use high registers. Also, we can use them as 18 temporaries to spill values into. 19 20 * In thumb mode, short, byte, and bool preferred alignments are currently set 21 to 4 to accommodate ISA restriction (i.e. add sp, #imm, imm must be multiple 22 of 4). 23 24 //===---------------------------------------------------------------------===// 25 26 Potential jumptable improvements: 27 28 * If we know function size is less than (1 << 16) * 2 bytes, we can use 16-bit 29 jumptable entries (e.g. (L1 - L2) >> 1). Or even smaller entries if the 30 function is even smaller. This also applies to ARM. 31 32 * Thumb jumptable codegen can improve given some help from the assembler. This 33 is what we generate right now: 34 35 .set PCRELV0, (LJTI1_0_0-(LPCRELL0+4)) 36 LPCRELL0: 37 mov r1, #PCRELV0 38 add r1, pc 39 ldr r0, [r0, r1] 40 mov pc, r0 41 .align 2 42 LJTI1_0_0: 43 .long LBB1_3 44 ... 45 46 Note there is another pc relative add that we can take advantage of. 47 add r1, pc, #imm_8 * 4 48 49 We should be able to generate: 50 51 LPCRELL0: 52 add r1, LJTI1_0_0 53 ldr r0, [r0, r1] 54 mov pc, r0 55 .align 2 56 LJTI1_0_0: 57 .long LBB1_3 58 59 if the assembler can translate the add to: 60 add r1, pc, #((LJTI1_0_0-(LPCRELL0+4))&0xfffffffc) 61 62 Note the assembler also does something similar to constpool load: 63 LPCRELL0: 64 ldr r0, LCPI1_0 65 => 66 ldr r0, pc, #((LCPI1_0-(LPCRELL0+4))&0xfffffffc) 67 68 69 //===---------------------------------------------------------------------===// 70 71 We compile the following: 72 73 define i16 @func_entry_2E_ce(i32 %i) { 74 switch i32 %i, label %bb12.exitStub [ 75 i32 0, label %bb4.exitStub 76 i32 1, label %bb9.exitStub 77 i32 2, label %bb4.exitStub 78 i32 3, label %bb4.exitStub 79 i32 7, label %bb9.exitStub 80 i32 8, label %bb.exitStub 81 i32 9, label %bb9.exitStub 82 ] 83 84 bb12.exitStub: 85 ret i16 0 86 87 bb4.exitStub: 88 ret i16 1 89 90 bb9.exitStub: 91 ret i16 2 92 93 bb.exitStub: 94 ret i16 3 95 } 96 97 into: 98 99 _func_entry_2E_ce: 100 mov r2, #1 101 lsl r2, r0 102 cmp r0, #9 103 bhi LBB1_4 @bb12.exitStub 104 LBB1_1: @newFuncRoot 105 mov r1, #13 106 tst r2, r1 107 bne LBB1_5 @bb4.exitStub 108 LBB1_2: @newFuncRoot 109 ldr r1, LCPI1_0 110 tst r2, r1 111 bne LBB1_6 @bb9.exitStub 112 LBB1_3: @newFuncRoot 113 mov r1, #1 114 lsl r1, r1, #8 115 tst r2, r1 116 bne LBB1_7 @bb.exitStub 117 LBB1_4: @bb12.exitStub 118 mov r0, #0 119 bx lr 120 LBB1_5: @bb4.exitStub 121 mov r0, #1 122 bx lr 123 LBB1_6: @bb9.exitStub 124 mov r0, #2 125 bx lr 126 LBB1_7: @bb.exitStub 127 mov r0, #3 128 bx lr 129 LBB1_8: 130 .align 2 131 LCPI1_0: 132 .long 642 133 134 135 gcc compiles to: 136 137 cmp r0, #9 138 @ lr needed for prologue 139 bhi L2 140 ldr r3, L11 141 mov r2, #1 142 mov r1, r2, asl r0 143 ands r0, r3, r2, asl r0 144 movne r0, #2 145 bxne lr 146 tst r1, #13 147 beq L9 148 L3: 149 mov r0, r2 150 bx lr 151 L9: 152 tst r1, #256 153 movne r0, #3 154 bxne lr 155 L2: 156 mov r0, #0 157 bx lr 158 L12: 159 .align 2 160 L11: 161 .long 642 162 163 164 GCC is doing a couple of clever things here: 165 1. It is predicating one of the returns. This isn't a clear win though: in 166 cases where that return isn't taken, it is replacing one condbranch with 167 two 'ne' predicated instructions. 168 2. It is sinking the shift of "1 << i" into the tst, and using ands instead of 169 tst. This will probably require whole function isel. 170 3. GCC emits: 171 tst r1, #256 172 we emit: 173 mov r1, #1 174 lsl r1, r1, #8 175 tst r2, r1 176 177 178 //===---------------------------------------------------------------------===// 179 180 When spilling in thumb mode and the sp offset is too large to fit in the ldr / 181 str offset field, we load the offset from a constpool entry and add it to sp: 182 183 ldr r2, LCPI 184 add r2, sp 185 ldr r2, [r2] 186 187 These instructions preserve the condition code which is important if the spill 188 is between a cmp and a bcc instruction. However, we can use the (potentially) 189 cheaper sequnce if we know it's ok to clobber the condition register. 190 191 add r2, sp, #255 * 4 192 add r2, #132 193 ldr r2, [r2, #7 * 4] 194 195 This is especially bad when dynamic alloca is used. The all fixed size stack 196 objects are referenced off the frame pointer with negative offsets. See 197 oggenc for an example. 198 199 200 //===---------------------------------------------------------------------===// 201 202 Poor codegen test/CodeGen/ARM/select.ll f7: 203 204 ldr r5, LCPI1_0 205 LPC0: 206 add r5, pc 207 ldr r6, LCPI1_1 208 ldr r2, LCPI1_2 209 mov r3, r6 210 mov lr, pc 211 bx r5 212 213 //===---------------------------------------------------------------------===// 214 215 Make register allocator / spiller smarter so we can re-materialize "mov r, imm", 216 etc. Almost all Thumb instructions clobber condition code. 217 218 //===---------------------------------------------------------------------===// 219 220 Add ldmia, stmia support. 221 222 //===---------------------------------------------------------------------===// 223 224 Thumb load / store address mode offsets are scaled. The values kept in the 225 instruction operands are pre-scale values. This probably ought to be changed 226 to avoid extra work when we convert Thumb2 instructions to Thumb1 instructions. 227 228 //===---------------------------------------------------------------------===// 229 230 We need to make (some of the) Thumb1 instructions predicable. That will allow 231 shrinking of predicated Thumb2 instructions. To allow this, we need to be able 232 to toggle the 's' bit since they do not set CPSR when they are inside IT blocks. 233 234 //===---------------------------------------------------------------------===// 235 236 Make use of hi register variants of cmp: tCMPhir / tCMPZhir. 237 238 //===---------------------------------------------------------------------===// 239 240 Thumb1 immediate field sometimes keep pre-scaled values. See 241 Thumb1RegisterInfo::eliminateFrameIndex. This is inconsistent from ARM and 242 Thumb2. 243 244 //===---------------------------------------------------------------------===// 245 246 Rather than having tBR_JTr print a ".align 2" and constant island pass pad it, 247 add a target specific ALIGN instruction instead. That way, GetInstSizeInBytes 248 won't have to over-estimate. It can also be used for loop alignment pass. 249 250 //===---------------------------------------------------------------------===// 251 252 We generate conditional code for icmp when we don't need to. This code: 253 254 int foo(int s) { 255 return s == 1; 256 } 257 258 produces: 259 260 foo: 261 cmp r0, #1 262 mov.w r0, #0 263 it eq 264 moveq r0, #1 265 bx lr 266 267 when it could use subs + adcs. This is GCC PR46975. 268
README-Thumb2.txt
1 //===---------------------------------------------------------------------===// 2 // Random ideas for the ARM backend (Thumb2 specific). 3 //===---------------------------------------------------------------------===// 4 5 Make sure jumptable destinations are below the jumptable in order to make use 6 of tbb / tbh. 7
README.txt
1 //===---------------------------------------------------------------------===// 2 // Random ideas for the ARM backend. 3 //===---------------------------------------------------------------------===// 4 5 Reimplement 'select' in terms of 'SEL'. 6 7 * We would really like to support UXTAB16, but we need to prove that the 8 add doesn't need to overflow between the two 16-bit chunks. 9 10 * Implement pre/post increment support. (e.g. PR935) 11 * Implement smarter constant generation for binops with large immediates. 12 13 A few ARMv6T2 ops should be pattern matched: BFI, SBFX, and UBFX 14 15 Interesting optimization for PIC codegen on arm-linux: 16 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=43129 17 18 //===---------------------------------------------------------------------===// 19 20 Crazy idea: Consider code that uses lots of 8-bit or 16-bit values. By the 21 time regalloc happens, these values are now in a 32-bit register, usually with 22 the top-bits known to be sign or zero extended. If spilled, we should be able 23 to spill these to a 8-bit or 16-bit stack slot, zero or sign extending as part 24 of the reload. 25 26 Doing this reduces the size of the stack frame (important for thumb etc), and 27 also increases the likelihood that we will be able to reload multiple values 28 from the stack with a single load. 29 30 //===---------------------------------------------------------------------===// 31 32 The constant island pass is in good shape. Some cleanups might be desirable, 33 but there is unlikely to be much improvement in the generated code. 34 35 1. There may be some advantage to trying to be smarter about the initial 36 placement, rather than putting everything at the end. 37 38 2. There might be some compile-time efficiency to be had by representing 39 consecutive islands as a single block rather than multiple blocks. 40 41 3. Use a priority queue to sort constant pool users in inverse order of 42 position so we always process the one closed to the end of functions 43 first. This may simply CreateNewWater. 44 45 //===---------------------------------------------------------------------===// 46 47 Eliminate copysign custom expansion. We are still generating crappy code with 48 default expansion + if-conversion. 49 50 //===---------------------------------------------------------------------===// 51 52 Eliminate one instruction from: 53 54 define i32 @_Z6slow4bii(i32 %x, i32 %y) { 55 %tmp = icmp sgt i32 %x, %y 56 %retval = select i1 %tmp, i32 %x, i32 %y 57 ret i32 %retval 58 } 59 60 __Z6slow4bii: 61 cmp r0, r1 62 movgt r1, r0 63 mov r0, r1 64 bx lr 65 => 66 67 __Z6slow4bii: 68 cmp r0, r1 69 movle r0, r1 70 bx lr 71 72 //===---------------------------------------------------------------------===// 73 74 Implement long long "X-3" with instructions that fold the immediate in. These 75 were disabled due to badness with the ARM carry flag on subtracts. 76 77 //===---------------------------------------------------------------------===// 78 79 More load / store optimizations: 80 1) Better representation for block transfer? This is from Olden/power: 81 82 fldd d0, [r4] 83 fstd d0, [r4, #+32] 84 fldd d0, [r4, #+8] 85 fstd d0, [r4, #+40] 86 fldd d0, [r4, #+16] 87 fstd d0, [r4, #+48] 88 fldd d0, [r4, #+24] 89 fstd d0, [r4, #+56] 90 91 If we can spare the registers, it would be better to use fldm and fstm here. 92 Need major register allocator enhancement though. 93 94 2) Can we recognize the relative position of constantpool entries? i.e. Treat 95 96 ldr r0, LCPI17_3 97 ldr r1, LCPI17_4 98 ldr r2, LCPI17_5 99 100 as 101 ldr r0, LCPI17 102 ldr r1, LCPI17+4 103 ldr r2, LCPI17+8 104 105 Then the ldr's can be combined into a single ldm. See Olden/power. 106 107 Note for ARM v4 gcc uses ldmia to load a pair of 32-bit values to represent a 108 double 64-bit FP constant: 109 110 adr r0, L6 111 ldmia r0, {r0-r1} 112 113 .align 2 114 L6: 115 .long -858993459 116 .long 1074318540 117 118 3) struct copies appear to be done field by field 119 instead of by words, at least sometimes: 120 121 struct foo { int x; short s; char c1; char c2; }; 122 void cpy(struct foo*a, struct foo*b) { *a = *b; } 123 124 llvm code (-O2) 125 ldrb r3, [r1, #+6] 126 ldr r2, [r1] 127 ldrb r12, [r1, #+7] 128 ldrh r1, [r1, #+4] 129 str r2, [r0] 130 strh r1, [r0, #+4] 131 strb r3, [r0, #+6] 132 strb r12, [r0, #+7] 133 gcc code (-O2) 134 ldmia r1, {r1-r2} 135 stmia r0, {r1-r2} 136 137 In this benchmark poor handling of aggregate copies has shown up as 138 having a large effect on size, and possibly speed as well (we don't have 139 a good way to measure on ARM). 140 141 //===---------------------------------------------------------------------===// 142 143 * Consider this silly example: 144 145 double bar(double x) { 146 double r = foo(3.1); 147 return x+r; 148 } 149 150 _bar: 151 stmfd sp!, {r4, r5, r7, lr} 152 add r7, sp, #8 153 mov r4, r0 154 mov r5, r1 155 fldd d0, LCPI1_0 156 fmrrd r0, r1, d0 157 bl _foo 158 fmdrr d0, r4, r5 159 fmsr s2, r0 160 fsitod d1, s2 161 faddd d0, d1, d0 162 fmrrd r0, r1, d0 163 ldmfd sp!, {r4, r5, r7, pc} 164 165 Ignore the prologue and epilogue stuff for a second. Note 166 mov r4, r0 167 mov r5, r1 168 the copys to callee-save registers and the fact they are only being used by the 169 fmdrr instruction. It would have been better had the fmdrr been scheduled 170 before the call and place the result in a callee-save DPR register. The two 171 mov ops would not have been necessary. 172 173 //===---------------------------------------------------------------------===// 174 175 Calling convention related stuff: 176 177 * gcc's parameter passing implementation is terrible and we suffer as a result: 178 179 e.g. 180 struct s { 181 double d1; 182 int s1; 183 }; 184 185 void foo(struct s S) { 186 printf("%g, %d\n", S.d1, S.s1); 187 } 188 189 'S' is passed via registers r0, r1, r2. But gcc stores them to the stack, and 190 then reload them to r1, r2, and r3 before issuing the call (r0 contains the 191 address of the format string): 192 193 stmfd sp!, {r7, lr} 194 add r7, sp, #0 195 sub sp, sp, #12 196 stmia sp, {r0, r1, r2} 197 ldmia sp, {r1-r2} 198 ldr r0, L5 199 ldr r3, [sp, #8] 200 L2: 201 add r0, pc, r0 202 bl L_printf$stub 203 204 Instead of a stmia, ldmia, and a ldr, wouldn't it be better to do three moves? 205 206 * Return an aggregate type is even worse: 207 208 e.g. 209 struct s foo(void) { 210 struct s S = {1.1, 2}; 211 return S; 212 } 213 214 mov ip, r0 215 ldr r0, L5 216 sub sp, sp, #12 217 L2: 218 add r0, pc, r0 219 @ lr needed for prologue 220 ldmia r0, {r0, r1, r2} 221 stmia sp, {r0, r1, r2} 222 stmia ip, {r0, r1, r2} 223 mov r0, ip 224 add sp, sp, #12 225 bx lr 226 227 r0 (and later ip) is the hidden parameter from caller to store the value in. The 228 first ldmia loads the constants into r0, r1, r2. The last stmia stores r0, r1, 229 r2 into the address passed in. However, there is one additional stmia that 230 stores r0, r1, and r2 to some stack location. The store is dead. 231 232 The llvm-gcc generated code looks like this: 233 234 csretcc void %foo(%struct.s* %agg.result) { 235 entry: 236 %S = alloca %struct.s, align 4 ; <%struct.s*> [#uses=1] 237 %memtmp = alloca %struct.s ; <%struct.s*> [#uses=1] 238 cast %struct.s* %S to sbyte* ; <sbyte*>:0 [#uses=2] 239 call void %llvm.memcpy.i32( sbyte* %0, sbyte* cast ({ double, int }* %C.0.904 to sbyte*), uint 12, uint 4 ) 240 cast %struct.s* %agg.result to sbyte* ; <sbyte*>:1 [#uses=2] 241 call void %llvm.memcpy.i32( sbyte* %1, sbyte* %0, uint 12, uint 0 ) 242 cast %struct.s* %memtmp to sbyte* ; <sbyte*>:2 [#uses=1] 243 call void %llvm.memcpy.i32( sbyte* %2, sbyte* %1, uint 12, uint 0 ) 244 ret void 245 } 246 247 llc ends up issuing two memcpy's (the first memcpy becomes 3 loads from 248 constantpool). Perhaps we should 1) fix llvm-gcc so the memcpy is translated 249 into a number of load and stores, or 2) custom lower memcpy (of small size) to 250 be ldmia / stmia. I think option 2 is better but the current register 251 allocator cannot allocate a chunk of registers at a time. 252 253 A feasible temporary solution is to use specific physical registers at the 254 lowering time for small (<= 4 words?) transfer size. 255 256 * ARM CSRet calling convention requires the hidden argument to be returned by 257 the callee. 258 259 //===---------------------------------------------------------------------===// 260 261 We can definitely do a better job on BB placements to eliminate some branches. 262 It's very common to see llvm generated assembly code that looks like this: 263 264 LBB3: 265 ... 266 LBB4: 267 ... 268 beq LBB3 269 b LBB2 270 271 If BB4 is the only predecessor of BB3, then we can emit BB3 after BB4. We can 272 then eliminate beq and and turn the unconditional branch to LBB2 to a bne. 273 274 See McCat/18-imp/ComputeBoundingBoxes for an example. 275 276 //===---------------------------------------------------------------------===// 277 278 Pre-/post- indexed load / stores: 279 280 1) We should not make the pre/post- indexed load/store transform if the base ptr 281 is guaranteed to be live beyond the load/store. This can happen if the base 282 ptr is live out of the block we are performing the optimization. e.g. 283 284 mov r1, r2 285 ldr r3, [r1], #4 286 ... 287 288 vs. 289 290 ldr r3, [r2] 291 add r1, r2, #4 292 ... 293 294 In most cases, this is just a wasted optimization. However, sometimes it can 295 negatively impact the performance because two-address code is more restrictive 296 when it comes to scheduling. 297 298 Unfortunately, liveout information is currently unavailable during DAG combine 299 time. 300 301 2) Consider spliting a indexed load / store into a pair of add/sub + load/store 302 to solve #1 (in TwoAddressInstructionPass.cpp). 303 304 3) Enhance LSR to generate more opportunities for indexed ops. 305 306 4) Once we added support for multiple result patterns, write indexed loads 307 patterns instead of C++ instruction selection code. 308 309 5) Use VLDM / VSTM to emulate indexed FP load / store. 310 311 //===---------------------------------------------------------------------===// 312 313 Implement support for some more tricky ways to materialize immediates. For 314 example, to get 0xffff8000, we can use: 315 316 mov r9, #&3f8000 317 sub r9, r9, #&400000 318 319 //===---------------------------------------------------------------------===// 320 321 We sometimes generate multiple add / sub instructions to update sp in prologue 322 and epilogue if the inc / dec value is too large to fit in a single immediate 323 operand. In some cases, perhaps it might be better to load the value from a 324 constantpool instead. 325 326 //===---------------------------------------------------------------------===// 327 328 GCC generates significantly better code for this function. 329 330 int foo(int StackPtr, unsigned char *Line, unsigned char *Stack, int LineLen) { 331 int i = 0; 332 333 if (StackPtr != 0) { 334 while (StackPtr != 0 && i < (((LineLen) < (32768))? (LineLen) : (32768))) 335 Line[i++] = Stack[--StackPtr]; 336 if (LineLen > 32768) 337 { 338 while (StackPtr != 0 && i < LineLen) 339 { 340 i++; 341 --StackPtr; 342 } 343 } 344 } 345 return StackPtr; 346 } 347 348 //===---------------------------------------------------------------------===// 349 350 This should compile to the mlas instruction: 351 int mlas(int x, int y, int z) { return ((x * y + z) < 0) ? 7 : 13; } 352 353 //===---------------------------------------------------------------------===// 354 355 At some point, we should triage these to see if they still apply to us: 356 357 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=19598 358 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=18560 359 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=27016 360 361 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11831 362 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11826 363 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11825 364 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11824 365 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11823 366 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=11820 367 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10982 368 369 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=10242 370 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9831 371 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9760 372 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9759 373 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9703 374 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9702 375 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=9663 376 377 http://www.inf.u-szeged.hu/gcc-arm/ 378 http://citeseer.ist.psu.edu/debus04linktime.html 379 380 //===---------------------------------------------------------------------===// 381 382 gcc generates smaller code for this function at -O2 or -Os: 383 384 void foo(signed char* p) { 385 if (*p == 3) 386 bar(); 387 else if (*p == 4) 388 baz(); 389 else if (*p == 5) 390 quux(); 391 } 392 393 llvm decides it's a good idea to turn the repeated if...else into a 394 binary tree, as if it were a switch; the resulting code requires -1 395 compare-and-branches when *p<=2 or *p==5, the same number if *p==4 396 or *p>6, and +1 if *p==3. So it should be a speed win 397 (on balance). However, the revised code is larger, with 4 conditional 398 branches instead of 3. 399 400 More seriously, there is a byte->word extend before 401 each comparison, where there should be only one, and the condition codes 402 are not remembered when the same two values are compared twice. 403 404 //===---------------------------------------------------------------------===// 405 406 More LSR enhancements possible: 407 408 1. Teach LSR about pre- and post- indexed ops to allow iv increment be merged 409 in a load / store. 410 2. Allow iv reuse even when a type conversion is required. For example, i8 411 and i32 load / store addressing modes are identical. 412 413 414 //===---------------------------------------------------------------------===// 415 416 This: 417 418 int foo(int a, int b, int c, int d) { 419 long long acc = (long long)a * (long long)b; 420 acc += (long long)c * (long long)d; 421 return (int)(acc >> 32); 422 } 423 424 Should compile to use SMLAL (Signed Multiply Accumulate Long) which multiplies 425 two signed 32-bit values to produce a 64-bit value, and accumulates this with 426 a 64-bit value. 427 428 We currently get this with both v4 and v6: 429 430 _foo: 431 smull r1, r0, r1, r0 432 smull r3, r2, r3, r2 433 adds r3, r3, r1 434 adc r0, r2, r0 435 bx lr 436 437 //===---------------------------------------------------------------------===// 438 439 This: 440 #include <algorithm> 441 std::pair<unsigned, bool> full_add(unsigned a, unsigned b) 442 { return std::make_pair(a + b, a + b < a); } 443 bool no_overflow(unsigned a, unsigned b) 444 { return !full_add(a, b).second; } 445 446 Should compile to: 447 448 _Z8full_addjj: 449 adds r2, r1, r2 450 movcc r1, #0 451 movcs r1, #1 452 str r2, [r0, #0] 453 strb r1, [r0, #4] 454 mov pc, lr 455 456 _Z11no_overflowjj: 457 cmn r0, r1 458 movcs r0, #0 459 movcc r0, #1 460 mov pc, lr 461 462 not: 463 464 __Z8full_addjj: 465 add r3, r2, r1 466 str r3, [r0] 467 mov r2, #1 468 mov r12, #0 469 cmp r3, r1 470 movlo r12, r2 471 str r12, [r0, #+4] 472 bx lr 473 __Z11no_overflowjj: 474 add r3, r1, r0 475 mov r2, #1 476 mov r1, #0 477 cmp r3, r0 478 movhs r1, r2 479 mov r0, r1 480 bx lr 481 482 //===---------------------------------------------------------------------===// 483 484 Some of the NEON intrinsics may be appropriate for more general use, either 485 as target-independent intrinsics or perhaps elsewhere in the ARM backend. 486 Some of them may also be lowered to target-independent SDNodes, and perhaps 487 some new SDNodes could be added. 488 489 For example, maximum, minimum, and absolute value operations are well-defined 490 and standard operations, both for vector and scalar types. 491 492 The current NEON-specific intrinsics for count leading zeros and count one 493 bits could perhaps be replaced by the target-independent ctlz and ctpop 494 intrinsics. It may also make sense to add a target-independent "ctls" 495 intrinsic for "count leading sign bits". Likewise, the backend could use 496 the target-independent SDNodes for these operations. 497 498 ARMv6 has scalar saturating and halving adds and subtracts. The same 499 intrinsics could possibly be used for both NEON's vector implementations of 500 those operations and the ARMv6 scalar versions. 501 502 //===---------------------------------------------------------------------===// 503 504 Split out LDR (literal) from normal ARM LDR instruction. Also consider spliting 505 LDR into imm12 and so_reg forms. This allows us to clean up some code. e.g. 506 ARMLoadStoreOptimizer does not need to look at LDR (literal) and LDR (so_reg) 507 while ARMConstantIslandPass only need to worry about LDR (literal). 508 509 //===---------------------------------------------------------------------===// 510 511 Constant island pass should make use of full range SoImm values for LEApcrel. 512 Be careful though as the last attempt caused infinite looping on lencod. 513 514 //===---------------------------------------------------------------------===// 515 516 Predication issue. This function: 517 518 extern unsigned array[ 128 ]; 519 int foo( int x ) { 520 int y; 521 y = array[ x & 127 ]; 522 if ( x & 128 ) 523 y = 123456789 & ( y >> 2 ); 524 else 525 y = 123456789 & y; 526 return y; 527 } 528 529 compiles to: 530 531 _foo: 532 and r1, r0, #127 533 ldr r2, LCPI1_0 534 ldr r2, [r2] 535 ldr r1, [r2, +r1, lsl #2] 536 mov r2, r1, lsr #2 537 tst r0, #128 538 moveq r2, r1 539 ldr r0, LCPI1_1 540 and r0, r2, r0 541 bx lr 542 543 It would be better to do something like this, to fold the shift into the 544 conditional move: 545 546 and r1, r0, #127 547 ldr r2, LCPI1_0 548 ldr r2, [r2] 549 ldr r1, [r2, +r1, lsl #2] 550 tst r0, #128 551 movne r1, r1, lsr #2 552 ldr r0, LCPI1_1 553 and r0, r1, r0 554 bx lr 555 556 it saves an instruction and a register. 557 558 //===---------------------------------------------------------------------===// 559 560 It might be profitable to cse MOVi16 if there are lots of 32-bit immediates 561 with the same bottom half. 562 563 //===---------------------------------------------------------------------===// 564 565 Robert Muth started working on an alternate jump table implementation that 566 does not put the tables in-line in the text. This is more like the llvm 567 default jump table implementation. This might be useful sometime. Several 568 revisions of patches are on the mailing list, beginning at: 569 http://lists.cs.uiuc.edu/pipermail/llvmdev/2009-June/022763.html 570 571 //===---------------------------------------------------------------------===// 572 573 Make use of the "rbit" instruction. 574 575 //===---------------------------------------------------------------------===// 576 577 Take a look at test/CodeGen/Thumb2/machine-licm.ll. ARM should be taught how 578 to licm and cse the unnecessary load from cp#1. 579 580 //===---------------------------------------------------------------------===// 581 582 The CMN instruction sets the flags like an ADD instruction, while CMP sets 583 them like a subtract. Therefore to be able to use CMN for comparisons other 584 than the Z bit, we'll need additional logic to reverse the conditionals 585 associated with the comparison. Perhaps a pseudo-instruction for the comparison, 586 with a post-codegen pass to clean up and handle the condition codes? 587 See PR5694 for testcase. 588 589 //===---------------------------------------------------------------------===// 590 591 Given the following on armv5: 592 int test1(int A, int B) { 593 return (A&-8388481)|(B&8388480); 594 } 595 596 We currently generate: 597 ldr r2, .LCPI0_0 598 and r0, r0, r2 599 ldr r2, .LCPI0_1 600 and r1, r1, r2 601 orr r0, r1, r0 602 bx lr 603 604 We should be able to replace the second ldr+and with a bic (i.e. reuse the 605 constant which was already loaded). Not sure what's necessary to do that. 606 607 //===---------------------------------------------------------------------===// 608 609 The code generated for bswap on armv4/5 (CPUs without rev) is less than ideal: 610 611 int a(int x) { return __builtin_bswap32(x); } 612 613 a: 614 mov r1, #255, 24 615 mov r2, #255, 16 616 and r1, r1, r0, lsr #8 617 and r2, r2, r0, lsl #8 618 orr r1, r1, r0, lsr #24 619 orr r0, r2, r0, lsl #24 620 orr r0, r0, r1 621 bx lr 622 623 Something like the following would be better (fewer instructions/registers): 624 eor r1, r0, r0, ror #16 625 bic r1, r1, #0xff0000 626 mov r1, r1, lsr #8 627 eor r0, r1, r0, ror #8 628 bx lr 629 630 A custom Thumb version would also be a slight improvement over the generic 631 version. 632 633 //===---------------------------------------------------------------------===// 634 635 Consider the following simple C code: 636 637 void foo(unsigned char *a, unsigned char *b, int *c) { 638 if ((*a | *b) == 0) *c = 0; 639 } 640 641 currently llvm-gcc generates something like this (nice branchless code I'd say): 642 643 ldrb r0, [r0] 644 ldrb r1, [r1] 645 orr r0, r1, r0 646 tst r0, #255 647 moveq r0, #0 648 streq r0, [r2] 649 bx lr 650 651 Note that both "tst" and "moveq" are redundant. 652 653 //===---------------------------------------------------------------------===// 654 655 When loading immediate constants with movt/movw, if there are multiple 656 constants needed with the same low 16 bits, and those values are not live at 657 the same time, it would be possible to use a single movw instruction, followed 658 by multiple movt instructions to rewrite the high bits to different values. 659 For example: 660 661 volatile store i32 -1, i32* inttoptr (i32 1342210076 to i32*), align 4, 662 !tbaa 663 !0 664 volatile store i32 -1, i32* inttoptr (i32 1342341148 to i32*), align 4, 665 !tbaa 666 !0 667 668 is compiled and optimized to: 669 670 movw r0, #32796 671 mov.w r1, #-1 672 movt r0, #20480 673 str r1, [r0] 674 movw r0, #32796 @ <= this MOVW is not needed, value is there already 675 movt r0, #20482 676 str r1, [r0] 677 678 //===---------------------------------------------------------------------===// 679 680 Improve codegen for select's: 681 if (x != 0) x = 1 682 if (x == 1) x = 1 683 684 ARM codegen used to look like this: 685 mov r1, r0 686 cmp r1, #1 687 mov r0, #0 688 moveq r0, #1 689 690 The naive lowering select between two different values. It should recognize the 691 test is equality test so it's more a conditional move rather than a select: 692 cmp r0, #1 693 movne r0, #0 694 695 Currently this is a ARM specific dag combine. We probably should make it into a 696 target-neutral one. 697 698 //===---------------------------------------------------------------------===// 699 700 Optimize unnecessary checks for zero with __builtin_clz/ctz. Those builtins 701 are specified to be undefined at zero, so portable code must check for zero 702 and handle it as a special case. That is unnecessary on ARM where those 703 operations are implemented in a way that is well-defined for zero. For 704 example: 705 706 int f(int x) { return x ? __builtin_clz(x) : sizeof(int)*8; } 707 708 should just be implemented with a CLZ instruction. Since there are other 709 targets, e.g., PPC, that share this behavior, it would be best to implement 710 this in a target-independent way: we should probably fold that (when using 711 "undefined at zero" semantics) to set the "defined at zero" bit and have 712 the code generator expand out the right code. 713
