README.txt
1//===-- README.txt - Notes for WebAssembly code gen -----------------------===// 2 3This WebAssembly backend is presently in a very early stage of development. 4The code should build and not break anything else, but don't expect a lot more 5at this point. 6 7For more information on WebAssembly itself, see the design documents: 8 * https://github.com/WebAssembly/design/blob/master/README.md 9 10The following documents contain some information on the planned semantics and 11binary encoding of WebAssembly itself: 12 * https://github.com/WebAssembly/design/blob/master/AstSemantics.md 13 * https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md 14 15The backend is built, tested and archived on the following waterfall: 16 https://build.chromium.org/p/client.wasm.llvm/console 17 18The backend's bringup is done using the GCC torture test suite first since it 19doesn't require C library support. Current known failures are in 20known_gcc_test_failures.txt, all other tests should pass. The waterfall will 21turn red if not. Once most of these pass, further testing will use LLVM's own 22test suite. The tests can be run locally using: 23 github.com/WebAssembly/experimental/blob/master/buildbot/torture_test.py 24 25Interesting work that remains to be done: 26* Write a pass to restructurize irreducible control flow. This needs to be done 27 before register allocation to be efficient, because it may duplicate basic 28 blocks and WebAssembly performs register allocation at a whole-function 29 level. Note that LLVM's GPU code has such a pass, but it linearizes control 30 flow (e.g. both sides of branches execute and are masked) which is undesirable 31 for WebAssembly. 32 33//===---------------------------------------------------------------------===// 34 35set_local instructions have a return value. We should (a) model this, 36and (b) write optimizations which take advantage of it. Keep in mind that 37many set_local instructions are implicit! 38 39//===---------------------------------------------------------------------===// 40 41Br, br_if, and tableswitch instructions can support having a value on the 42expression stack across the jump (sometimes). We should (a) model this, and 43(b) extend the stackifier to utilize it. 44 45//===---------------------------------------------------------------------===// 46 47The min/max operators aren't exactly a<b?a:b because of NaN and negative zero 48behavior. The ARM target has the same kind of min/max instructions and has 49implemented optimizations for them; we should do similar optimizations for 50WebAssembly. 51 52//===---------------------------------------------------------------------===// 53 54AArch64 runs SeparateConstOffsetFromGEPPass, followed by EarlyCSE and LICM. 55Would these be useful to run for WebAssembly too? Also, it has an option to 56run SimplifyCFG after running the AtomicExpand pass. Would this be useful for 57us too? 58 59//===---------------------------------------------------------------------===// 60 61When is it profitable to set isAsCheapAsAMove on instructions in WebAssembly? 62 63//===---------------------------------------------------------------------===// 64 65Register stackification uses the EXPR_STACK physical register to impose 66ordering dependencies on instructions with stack operands. This is pessimistic; 67we should consider alternate ways to model stack dependencies. 68 69//===---------------------------------------------------------------------===// 70 71Lots of things could be done in WebAssemblyTargetTransformInfo.cpp. Similarly, 72there are numerous optimization-related hooks that can be overridden in 73WebAssemblyTargetLowering. 74 75//===---------------------------------------------------------------------===// 76 77Instead of the OptimizeReturned pass, which should consider preserving the 78"returned" attribute through to MachineInstrs and extending the StoreResults 79pass to do this optimization on calls too. That would also let the 80WebAssemblyPeephole pass clean up dead defs for such calls, as it does for 81stores. 82 83//===---------------------------------------------------------------------===// 84 85Memset/memcpy/memmove should be marked with the "returned" attribute somehow, 86even when they are translated through intrinsics. 87 88//===---------------------------------------------------------------------===// 89