[llvm-bugs] [Bug 42973] New: [WebAssembly] SIMD bit-shifts can remove unroll thanks to V8 fixes

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=42973

            Bug ID: 42973
           Summary: [WebAssembly] SIMD bit-shifts can remove unroll thanks
                    to V8 fixes
           Product: new-bugs
           Version: trunk
          Hardware: All
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: bvibber at wikimedia.org
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

Created attachment 22370
  --> https://bugs.llvm.org/attachment.cgi?id=22370&action=edit
Patch that removes this

This bit in WebAssemblyTargetLowering::LowerShift

-  // Expand all vector shifts until V8 fixes its implementation
-  // TODO: remove this once V8 is fixed
-  if (!Subtarget->hasUnimplementedSIMD128())
-    return unrollVectorShift(Op, DAG);

can be removed since https://chromium-review.googlesource.com/c/v8/v8/+/1722198
fixed non-constant SIMD bit-shifts.

This gives me a speedup of several percent in dav1d AV1 decoder on my
WebAssembly SIMD branch running in Chrome Canary.

Attached patch reapplies https://reviews.llvm.org/D58800 which was accidentally
merged early a few months ago, then reverted.

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