[all-commits] [llvm/llvm-project] 10b781: Mark type test intrinsics as speculatable to fix i...

[Teresa Johnson via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 10b781fb033ee8d8a02e1050976b3832ad50542a
      https://github.com/llvm/llvm-project/commit/10b781fb033ee8d8a02e1050976b3832ad50542a
  Author: Teresa Johnson <tejohnson at google.com>
  Date:   2021-04-23 (Fri, 23 Apr 2021)

  Changed paths:
    M llvm/include/llvm/IR/Intrinsics.td
    M llvm/test/Transforms/Inline/ephemeral.ll
    M llvm/test/Transforms/SimplifyCFG/no-md-sink.ll

  Log Message:
  -----------
  Mark type test intrinsics as speculatable to fix inline cost

There is already code in InlineCost.cpp to identify and ignore ephemeral
values (llvm.assume intrinsics and other side-effect free instructions
only feeding the assumes). However, because llvm.type.test intrinsics
were not marked speculatable, they and any instructions specifically
feeding the type test (typically a bitcast) were being counted towards
the instruction cost when inlining. This was causing profile matching
issues in some cases when enabling -fwhole-program-vtables for whole
program devirtualization.

According to the language reference, the speculatable attribute means:
"the function does not have any effects besides calculating its result
and does not have undefined behavior". I see no reason why type tests
cannot be marked with this attribute.

There are 2 test changes:

llvm/test/Transforms/Inline/ephemeral.ll: I added a type test intrinsic
here to verify the fix. Also, I found the test was not actually testing
what it originally intended. Many of the existing instructions were
optimized away by -Oz, and the cost of inlining was negative due to the
benefit of removing the call. So I changed the test to simply invoke the
inline pass and check the number of instructions computed by InlineCost.
I also fixed an instruction that was not actually used anywhere.

llvm/test/Transforms/SimplifyCFG/no-md-sink.ll needed to be made more
robust to code changes that reordered the metadata.

Differential Revision: https://reviews.llvm.org/D101180