[all-commits] [llvm/llvm-project] af010e: Metadata: Optimize metadata queries (#70700)

[Matthias Braun via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: af010e79163b373e49fb45bf39878fd35d8dcd49
      https://github.com/llvm/llvm-project/commit/af010e79163b373e49fb45bf39878fd35d8dcd49
  Author: Matthias Braun <matze at braunis.de>
  Date:   2023-10-31 (Tue, 31 Oct 2023)

  Changed paths:
    M llvm/include/llvm/IR/Instruction.h
    M llvm/include/llvm/IR/Value.h
    M llvm/lib/IR/Metadata.cpp

  Log Message:
  -----------
  Metadata: Optimize metadata queries (#70700)

Optimize metadata query code:
- Avoid `DenseMap::operator[]` in situations where it is known that the
key exists in the map. Instead use `DenseMap::at()`/
`DenseMap::find()->second`. This avoids code-bloat and bad inlining
decisions for the unused insertion/growing code in `operator[]`.
- Avoid a redundant `Value::hasMetadata()` check.
- Move the `KindID == LLVMContext::MD_dbg` case to
`Instruction::getMetadata` and check it first assuming that it can be
constant folded after inlining in many situations.

The motivation for this change is a regression triggered by
e3cf80c5c1fe55efd8216575ccadea0ab087e79c which could attributed to the
compiler inlining the insertion part of `DenseMap::operator[]` in more
cases while unbeknownst to a compiler (without PGO) that code is never
used in this context. This change improves performance and eliminates
difference before and after that change in my measurements.