[PATCH] D122167: Compiler: Remove empty fallback definition for LLVM_BUILTIN_UNREACHABLE

[Duncan P. N. Exon Smith via Phabricator via llvm-commits]

dexonsmith created this revision.
dexonsmith added reviewers: aeubanks, mehdi_amini.
Herald added a project: All.
dexonsmith requested review of this revision.
Herald added a project: LLVM.

`llvm_unreachable()` and `LLVM_ASSUME_ALIGNED` use
`defined(LLVM_BUILTIN_UNREACHABLE)` to check whether it has a
definition. Remove the fallback added in 26827337dff26ba3 <https://reviews.llvm.org/rG26827337dff26ba3450721f880d4c6caaf2a8219> (as a drive-by
when updating the GCC logic) and add a comment to prevent future
mistakes.


Repository:
  rG LLVM Github Monorepo

https://reviews.llvm.org/D122167

Files:
  llvm/include/llvm/Support/Compiler.h


Index: llvm/include/llvm/Support/Compiler.h
===================================================================
--- llvm/include/llvm/Support/Compiler.h
+++ llvm/include/llvm/Support/Compiler.h
@@ -321,12 +321,14 @@
 /// LLVM_BUILTIN_UNREACHABLE - On compilers which support it, expands
 /// to an expression which states that it is undefined behavior for the
 /// compiler to reach this point.  Otherwise is not defined.
+///
+/// '#else' is intentionally left out so that other macro logic (e.g.,
+/// LLVM_ASSUME_ALIGNED and llvm_unreachable()) can detect whether
+/// LLVM_BUILTIN_UNREACHABLE has a definition.
 #if __has_builtin(__builtin_unreachable) || defined(__GNUC__)
 # define LLVM_BUILTIN_UNREACHABLE __builtin_unreachable()
 #elif defined(_MSC_VER)
 # define LLVM_BUILTIN_UNREACHABLE __assume(false)
-#else
-# define LLVM_BUILTIN_UNREACHABLE
 #endif
 
 /// LLVM_BUILTIN_TRAP - On compilers which support it, expands to an expression


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