[PATCH] D136805: [NFC] Make format() more amenable to format attributes

[Félix Cloutier via Phabricator via llvm-commits]

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Second take on https://reviews.llvm.org/rGfb1e90ef07fec0d64a05c0b6d41117a5ea3e8344, which was pulled out because it broke GCC-based builds. The offending code was:

  constexpr uint64_t specifierBit(char C) { return 1 << (C - 0x40); }

The issue is that the shift expression has type `int`, but the shift value has range 0..<0x40. Shifting `(int)1` by more than 30 is undefined behavior.

With GCC, this manifested with test failures, and when sanitizers were enabled, with sanitizer traps. However, because the function is `constexpr`, Clang chose to evaluate it at compile-time even when when its execution was undefined. (Both Clang and GCC reject calls to `specifierBit` and its `specifierMask` caller when using `constinit`.) Unfortunately, somehow, the result was good enough to pass all tests when built with clang; and since the undefined behavior was evaluated at build time, UBSan wouldn't catch it.

I have reproduced the problem with a GCC build of LLVM and verified that this change fixes it:

  constexpr uint64_t specifierBit(char C) { return (uint64_t)1 << (C - 0x40); }


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D136805

Files:
  llvm/include/llvm/Support/Format.h
  llvm/lib/Support/CMakeLists.txt
  llvm/lib/Support/Format.cpp
  llvm/lib/TableGen/SetTheory.cpp
  llvm/unittests/Support/CMakeLists.txt
  llvm/unittests/Support/FormatChkTest.cpp
  llvm/utils/TableGen/GlobalISel/GIMatchDag.cpp

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