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 When spilling in thumb mode and the sp offset is too large to fit in the ldr / 180 str offset field, we load the offset from a constpool entry and add it to sp: 181 182 ldr r2, LCPI 183 add r2, sp 184 ldr r2, [r2] 185 186 These instructions preserve the condition code which is important if the spill 187 is between a cmp and a bcc instruction. However, we can use the (potentially) 188 cheaper sequnce if we know it's ok to clobber the condition register. 189 190 add r2, sp, #255 * 4 191 add r2, #132 192 ldr r2, [r2, #7 * 4] 193 194 This is especially bad when dynamic alloca is used. The all fixed size stack 195 objects are referenced off the frame pointer with negative offsets. See 196 oggenc for an example. 197 198 //===---------------------------------------------------------------------===// 199 200 Poor codegen test/CodeGen/ARM/select.ll f7: 201 202 ldr r5, LCPI1_0 203 LPC0: 204 add r5, pc 205 ldr r6, LCPI1_1 206 ldr r2, LCPI1_2 207 mov r3, r6 208 mov lr, pc 209 bx r5 210 211 //===---------------------------------------------------------------------===// 212 213 Make register allocator / spiller smarter so we can re-materialize "mov r, imm", 214 etc. Almost all Thumb instructions clobber condition code. 215 216 //===---------------------------------------------------------------------===// 217 218 Thumb load / store address mode offsets are scaled. The values kept in the 219 instruction operands are pre-scale values. This probably ought to be changed 220 to avoid extra work when we convert Thumb2 instructions to Thumb1 instructions. 221 222 //===---------------------------------------------------------------------===// 223 224 We need to make (some of the) Thumb1 instructions predicable. That will allow 225 shrinking of predicated Thumb2 instructions. To allow this, we need to be able 226 to toggle the 's' bit since they do not set CPSR when they are inside IT blocks. 227 228 //===---------------------------------------------------------------------===// 229 230 Make use of hi register variants of cmp: tCMPhir / tCMPZhir. 231 232 //===---------------------------------------------------------------------===// 233 234 Thumb1 immediate field sometimes keep pre-scaled values. See 235 ThumbRegisterInfo::eliminateFrameIndex. This is inconsistent from ARM and 236 Thumb2. 237 238 //===---------------------------------------------------------------------===// 239 240 Rather than having tBR_JTr print a ".align 2" and constant island pass pad it, 241 add a target specific ALIGN instruction instead. That way, GetInstSizeInBytes 242 won't have to over-estimate. It can also be used for loop alignment pass. 243 244 //===---------------------------------------------------------------------===// 245 246 We generate conditional code for icmp when we don't need to. This code: 247 248 int foo(int s) { 249 return s == 1; 250 } 251 252 produces: 253 254 foo: 255 cmp r0, #1 256 mov.w r0, #0 257 it eq 258 moveq r0, #1 259 bx lr 260 261 when it could use subs + adcs. This is GCC PR46975. 262
