diff options
Diffstat (limited to 'llvm')
| -rw-r--r-- | llvm/lib/Target/WebAssembly/README.txt | 17 | ||||
| -rw-r--r-- | llvm/lib/Target/WebAssembly/WebAssemblyFixIrreducibleControlFlow.cpp | 4 |
2 files changed, 13 insertions, 8 deletions
diff --git a/llvm/lib/Target/WebAssembly/README.txt b/llvm/lib/Target/WebAssembly/README.txt index d6038f45479..404ea2ade0f 100644 --- a/llvm/lib/Target/WebAssembly/README.txt +++ b/llvm/lib/Target/WebAssembly/README.txt @@ -22,14 +22,6 @@ 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: https://github.com/WebAssembly/waterfall/blob/master/src/compile_torture_tests.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. - //===---------------------------------------------------------------------===// Br, br_if, and br_table instructions can support having a value on the @@ -112,3 +104,12 @@ just let that value be the exit value of the outermost block, rather than needing an explicit return operation. //===---------------------------------------------------------------------===// + +Many cases of irreducible control flow could be transformed more optimally +than via the transform in WebAssemblyFixIrreducibleControlFlow.cpp. + +It may also be worthwhile to do transforms before register coloring, +particularly when duplicating code, to allow register coloring to be aware of +the duplication. + +//===---------------------------------------------------------------------===// diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyFixIrreducibleControlFlow.cpp b/llvm/lib/Target/WebAssembly/WebAssemblyFixIrreducibleControlFlow.cpp index be77222f873..1a7716a5d94 100644 --- a/llvm/lib/Target/WebAssembly/WebAssemblyFixIrreducibleControlFlow.cpp +++ b/llvm/lib/Target/WebAssembly/WebAssemblyFixIrreducibleControlFlow.cpp @@ -13,6 +13,10 @@ /// loops; they appear as CFG cycles that are not recorded in MachineLoopInfo /// due to being unnatural. /// +/// Note that LLVM has a generic pass that lowers irreducible control flow, but +/// it linearizes control flow, turning diamonds into two triangles, which is +/// both unnecessary and undesirable for WebAssembly. +/// /// TODO: The transformation implemented here handles all irreducible control /// flow, without exponential code-size expansion, though it does so by creating /// inefficient code in many cases. Ideally, we should add other |
