README.txt
1 //===-- README.txt - Notes for WebAssembly code gen -----------------------===// 2 3 This WebAssembly backend is presently under development. 4 5 The most notable feature which is not yet stable is the ".o" file format. 6 ".o" file support is needed for many common ways of using LLVM, such as 7 using it through "clang -c", so this backend is not yet considered widely 8 usable. However, this backend is usable within some language toolchain 9 packages: 10 11 Emscripten provides a C/C++ compilation environment that includes standard 12 libraries, tools, and packaging for producing WebAssembly applications that 13 can run in browsers and other environments. For more information, see the 14 Emscripten documentation in general, and this page in particular: 15 16 * https://github.com/kripken/emscripten/wiki/New-WebAssembly-Backend 17 18 Rust provides WebAssembly support integrated into Cargo. There are two 19 main options: 20 - wasm32-unknown-unknown, which provides a relatively minimal environment 21 that has an emphasis on being "native" 22 - wasm32-unknown-emscripten, which uses Emscripten internally and 23 provides standard C/C++ libraries, filesystem emulation, GL and SDL 24 bindings 25 For more information, see: 26 * https://www.hellorust.com/ 27 28 29 This backend does not yet support debug info. Full DWARF support needs a 30 design for how DWARF should be represented in WebAssembly. Sourcemap support 31 has an existing design and some corresponding browser implementations, so it 32 just needs implementing in LLVM. 33 34 Work-in-progress documentation for the ".o" file format is here: 35 36 * https://github.com/WebAssembly/tool-conventions/blob/master/Linking.md 37 38 A corresponding linker implementation is also under development: 39 40 * https://lld.llvm.org/WebAssembly.html 41 42 For more information on WebAssembly itself, see the home page: 43 * https://webassembly.github.io/ 44 45 The following documents contain some information on the semantics and binary 46 encoding of WebAssembly itself: 47 * https://github.com/WebAssembly/design/blob/master/Semantics.md 48 * https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md 49 50 The backend is built, tested and archived on the following waterfall: 51 https://wasm-stat.us 52 53 The backend's bringup is done in part by using the GCC torture test suite, since 54 it doesn't require C library support. Current known failures are in 55 known_gcc_test_failures.txt, all other tests should pass. The waterfall will 56 turn red if not. Once most of these pass, further testing will use LLVM's own 57 test suite. The tests can be run locally using: 58 https://github.com/WebAssembly/waterfall/blob/master/src/compile_torture_tests.py 59 60 Some notes on ways that the generated code could be improved follow: 61 62 //===---------------------------------------------------------------------===// 63 64 Br, br_if, and br_table instructions can support having a value on the value 65 stack across the jump (sometimes). We should (a) model this, and (b) extend 66 the stackifier to utilize it. 67 68 //===---------------------------------------------------------------------===// 69 70 The min/max instructions aren't exactly a<b?a:b because of NaN and negative zero 71 behavior. The ARM target has the same kind of min/max instructions and has 72 implemented optimizations for them; we should do similar optimizations for 73 WebAssembly. 74 75 //===---------------------------------------------------------------------===// 76 77 AArch64 runs SeparateConstOffsetFromGEPPass, followed by EarlyCSE and LICM. 78 Would these be useful to run for WebAssembly too? Also, it has an option to 79 run SimplifyCFG after running the AtomicExpand pass. Would this be useful for 80 us too? 81 82 //===---------------------------------------------------------------------===// 83 84 Register stackification uses the VALUE_STACK physical register to impose 85 ordering dependencies on instructions with stack operands. This is pessimistic; 86 we should consider alternate ways to model stack dependencies. 87 88 //===---------------------------------------------------------------------===// 89 90 Lots of things could be done in WebAssemblyTargetTransformInfo.cpp. Similarly, 91 there are numerous optimization-related hooks that can be overridden in 92 WebAssemblyTargetLowering. 93 94 //===---------------------------------------------------------------------===// 95 96 Instead of the OptimizeReturned pass, which should consider preserving the 97 "returned" attribute through to MachineInstrs and extending the StoreResults 98 pass to do this optimization on calls too. That would also let the 99 WebAssemblyPeephole pass clean up dead defs for such calls, as it does for 100 stores. 101 102 //===---------------------------------------------------------------------===// 103 104 Consider implementing optimizeSelect, optimizeCompareInstr, optimizeCondBranch, 105 optimizeLoadInstr, and/or getMachineCombinerPatterns. 106 107 //===---------------------------------------------------------------------===// 108 109 Find a clean way to fix the problem which leads to the Shrink Wrapping pass 110 being run after the WebAssembly PEI pass. 111 112 //===---------------------------------------------------------------------===// 113 114 When setting multiple local variables to the same constant, we currently get 115 code like this: 116 117 i32.const $4=, 0 118 i32.const $3=, 0 119 120 It could be done with a smaller encoding like this: 121 122 i32.const $push5=, 0 123 tee_local $push6=, $4=, $pop5 124 copy_local $3=, $pop6 125 126 //===---------------------------------------------------------------------===// 127 128 WebAssembly registers are implicitly initialized to zero. Explicit zeroing is 129 therefore often redundant and could be optimized away. 130 131 //===---------------------------------------------------------------------===// 132 133 Small indices may use smaller encodings than large indices. 134 WebAssemblyRegColoring and/or WebAssemblyRegRenumbering should sort registers 135 according to their usage frequency to maximize the usage of smaller encodings. 136 137 //===---------------------------------------------------------------------===// 138 139 Many cases of irreducible control flow could be transformed more optimally 140 than via the transform in WebAssemblyFixIrreducibleControlFlow.cpp. 141 142 It may also be worthwhile to do transforms before register coloring, 143 particularly when duplicating code, to allow register coloring to be aware of 144 the duplication. 145 146 //===---------------------------------------------------------------------===// 147 148 WebAssemblyRegStackify could use AliasAnalysis to reorder loads and stores more 149 aggressively. 150 151 //===---------------------------------------------------------------------===// 152 153 WebAssemblyRegStackify is currently a greedy algorithm. This means that, for 154 example, a binary operator will stackify with its user before its operands. 155 However, if moving the binary operator to its user moves it to a place where 156 its operands can't be moved to, it would be better to leave it in place, or 157 perhaps move it up, so that it can stackify its operands. A binary operator 158 has two operands and one result, so in such cases there could be a net win by 159 preferring the operands. 160 161 //===---------------------------------------------------------------------===// 162 163 Instruction ordering has a significant influence on register stackification and 164 coloring. Consider experimenting with the MachineScheduler (enable via 165 enableMachineScheduler) and determine if it can be configured to schedule 166 instructions advantageously for this purpose. 167 168 //===---------------------------------------------------------------------===// 169 170 WebAssemblyRegStackify currently assumes that the stack must be empty after 171 an instruction with no return values, however wasm doesn't actually require 172 this. WebAssemblyRegStackify could be extended, or possibly rewritten, to take 173 full advantage of what WebAssembly permits. 174 175 //===---------------------------------------------------------------------===// 176 177 Add support for mergeable sections in the Wasm writer, such as for strings and 178 floating-point constants. 179 180 //===---------------------------------------------------------------------===// 181 182 The function @dynamic_alloca_redzone in test/CodeGen/WebAssembly/userstack.ll 183 ends up with a tee_local in its prolog which has an unused result, requiring 184 an extra drop: 185 186 get_global $push8=, 0 187 tee_local $push9=, 1, $pop8 188 drop $pop9 189 [...] 190 191 The prologue code initially thinks it needs an FP register, but later it 192 turns out to be unneeded, so one could either approach this by being more 193 clever about not inserting code for an FP in the first place, or optimizing 194 away the copy later. 195 196 //===---------------------------------------------------------------------===// 197
