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//===-- README.txt - Notes for WebAssembly code gen -----------------------===// This WebAssembly backend is presently in a very early stage of development. The code should build and not break anything else, but don't expect a lot more at this point. For more information on WebAssembly itself, see the design documents: * https://github.com/WebAssembly/design/blob/master/README.md The following documents contain some information on the planned semantics and binary encoding of WebAssembly itself: * https://github.com/WebAssembly/design/blob/master/AstSemantics.md * https://github.com/WebAssembly/design/blob/master/BinaryEncoding.md The backend is built, tested and archived on the following waterfall: https://build.chromium.org/p/client.wasm.llvm/console The backend's bringup is done using the GCC torture test suite first since it doesn't require C library support. Current known failures are in known_gcc_test_failures.txt, all other tests should pass. The waterfall will turn red if not. Once most of these pass, further testing will use LLVM's own test suite. The tests can be run locally using: github.com/WebAssembly/experimental/blob/master/buildbot/torture_test.py Interesting work that remains to be done: * Write a pass to restructurize irreducible control flow. This needs to be done before register allocation to be efficient, because it may duplicate basic blocks and WebAssembly performs register allocation at a whole-function level. Note that LLVM's GPU code has such a pass, but it linearizes control flow (e.g. both sides of branches execute and are masked) which is undesirable for WebAssembly. //===---------------------------------------------------------------------===// set_local instructions have a return value. We should (a) model this, and (b) write optimizations which take advantage of it. Keep in mind that many set_local instructions are implicit! //===---------------------------------------------------------------------===// Br, br_if, and tableswitch instructions can support having a value on the expression stack across the jump (sometimes). We should (a) model this, and (b) extend the stackifier to utilize it. //===---------------------------------------------------------------------===// The min/max operators aren't exactly a<b?a:b because of NaN and negative zero behavior. The ARM target has the same kind of min/max instructions and has implemented optimizations for them; we should do similar optimizations for WebAssembly. //===---------------------------------------------------------------------===// AArch64 runs SeparateConstOffsetFromGEPPass, followed by EarlyCSE and LICM. Would these be useful to run for WebAssembly too? Also, it has an option to run SimplifyCFG after running the AtomicExpand pass. Would this be useful for us too? //===---------------------------------------------------------------------===// When is it profitable to set isAsCheapAsAMove on instructions in WebAssembly? //===---------------------------------------------------------------------===// Register stackification uses the EXPR_STACK physical register to impose ordering dependencies on instructions with stack operands. This is pessimistic; we should consider alternate ways to model stack dependencies. //===---------------------------------------------------------------------===// Lots of things could be done in WebAssemblyTargetTransformInfo.cpp. Similarly, there are numerous optimization-related hooks that can be overridden in WebAssemblyTargetLowering. //===---------------------------------------------------------------------===// Instead of the OptimizeReturned pass, which should consider preserving the "returned" attribute through to MachineInstrs and extending the StoreResults pass to do this optimization on calls too. That would also let the WebAssemblyPeephole pass clean up dead defs for such calls, as it does for stores. //===---------------------------------------------------------------------===// Memset/memcpy/memmove should be marked with the "returned" attribute somehow, even when they are translated through intrinsics. //===---------------------------------------------------------------------===// Consider implementing optimizeSelect, optimizeCompareInstr, optimizeCondBranch, optimizeLoadInstr, and/or getMachineCombinerPatterns. //===---------------------------------------------------------------------===// Find a clean way to fix the problem which leads to the Shrink Wrapping pass being run after the WebAssembly PEI pass. //===---------------------------------------------------------------------===//