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

[via cfe-commits]

https://github.com/AnonMiraj created https://github.com/llvm/llvm-project/pull/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


>From fcf318165e95224d26c37b0da81f1c6bb0f6399c Mon Sep 17 00:00:00 2001
From: Anonmiraj <ezzibrahimx at gmail.com>
Date: Sat, 11 Jul 2026 02:51:32 +0300
Subject: [PATCH] [clang] Skip macro source-location check for names that can't
 warn

---
 clang/lib/Lex/PPDirectives.cpp | 18 +++++++++---------
 1 file changed, 9 insertions(+), 9 deletions(-)

diff --git a/clang/lib/Lex/PPDirectives.cpp b/clang/lib/Lex/PPDirectives.cpp
index eb21a510dcf83..0323743ad9b20 100644
--- a/clang/lib/Lex/PPDirectives.cpp
+++ b/clang/lib/Lex/PPDirectives.cpp
@@ -396,16 +396,16 @@ 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