[WebAssembly] Enable atomic expansion for unsupported atomicrmws

Summary:
Wasm does not have direct counterparts to some of LLVM IR's atomicrmw
instructions (min, max, umin, umax, and nand). This enables atomic
expansion using cmpxchg instruction within a loop for those atomicrmw
instructions.

Reviewers: dschuff

Subscribers: sbc100, jgravelle-google, sunfish, llvm-commits

Differential Revision: https://reviews.llvm.org/D49440

llvm-svn: 339084

1 files changed, 5 insertions, 4 deletions

diff --git a/llvm/lib/Target/WebAssembly/WebAssemblyInstrAtomics.td b/llvm/lib/Target/WebAssembly/WebAssemblyInstrAtomics.td
index ba6800e005d..2c66efe09d8 100644
--- a/llvm/lib/Target/WebAssembly/WebAssemblyInstrAtomics.td
+++ b/llvm/lib/Target/WebAssembly/WebAssemblyInstrAtomics.td
@@ -660,10 +660,11 @@ defm : BinRMWTruncExtPattern<
 // Atomic ternary read-modify-writes
 //===----------------------------------------------------------------------===//
 
-// TODO LLVM IR's cmpxchg instruction returns a pair of {loaded value,
-// success flag}. When we use a success flag or both values, we can't make use
-// of truncate/extend versions of instructions for now, which is suboptimal. Add
-// selection rules for those cases too.
+// TODO LLVM IR's cmpxchg instruction returns a pair of {loaded value, success
+// flag}. When we use the success flag or both values, we can't make use of i64
+// truncate/extend versions of instructions for now, which is suboptimal.
+// Consider adding a pass after instruction selection that optimizes this case
+// if it is frequent.
 
 let Defs = [ARGUMENTS] in {