[clang] c42c7b4 - [clang][NFC] Skip macro source-location check for names that can't warn (#208862)

[via cfe-commits]

Author: Anonmiraj
Date: 2026-07-13T16:23:50+03:00
New Revision: c42c7b47a634ab3f4455b0e8b17f2bbc3be022af

URL: https://github.com/llvm/llvm-project/commit/c42c7b47a634ab3f4455b0e8b17f2bbc3be022af
DIFF: https://github.com/llvm/llvm-project/commit/c42c7b47a634ab3f4455b0e8b17f2bbc3be022af.diff

LOG: [clang][NFC] Skip macro source-location check for names that can't warn (#208862)

While profiling Linux headers, I noticed that
`Preprocessor::CheckMacroName` performs expensive source-location
queries for every macro, even when no diagnostic is possible. We can
avoid this overhead by only performing these checks when the macro name
is actually a reserved identifier or keyword, skipping them entirely for
ordinary macros.

|  | basket Ir |
| --- | --- |
| before | 10,690,004,315 |
| after | 10,649,181,540 |
| **Δ** | **−40.8M (−0.38%)** |

this is a follow up to #137306 and #139492

Added: 
    

Modified: 
    clang/lib/Lex/PPDirectives.cpp

Removed: 
    


################################################################################
diff  --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index 76ea6b6a6e19b..c161f6a03593e 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -396,16 +396,15 @@ bool Preprocessor::CheckMacroName(Token &MacroNameTok, MacroUse isDefineUndef,
   // Macro names with reserved identifiers are accepted if built-in or passed
   // through the command line (the later may be present if -dD was used to
   // generate the preprocessed file).
-  // NB: isInPredefinedFile() is relatively expensive, so keep it at the end
-  // of the condition.
-  if (!SourceMgr.isInSystemHeader(MacroNameLoc) &&
+  // NB: isInPredefinedFile() (via getPresumedLoc) is relatively expensive, so
+  // only run it for names that can actually warn.
+  MacroDiag D = MD_NoWarn;
+  if (isDefineUndef == MU_Define) {
+    D = shouldWarnOnMacroDef(*this, II);
+  } else if (isDefineUndef == MU_Undef)
+    D = shouldWarnOnMacroUndef(*this, II);
+  if (D != MD_NoWarn && !SourceMgr.isInSystemHeader(MacroNameLoc) &&
       !SourceMgr.isInPredefinedFile(MacroNameLoc)) {
-    MacroDiag D = MD_NoWarn;
-    if (isDefineUndef == MU_Define) {
-      D = shouldWarnOnMacroDef(*this, II);
-    }
-    else if (isDefineUndef == MU_Undef)
-      D = shouldWarnOnMacroUndef(*this, II);
     if (D == MD_KeywordDef) {
       // We do not want to warn on some patterns widely used in configuration
       // scripts.  This requires analyzing next tokens, so do not issue warnings