[llvm] -fsanitize=function: fix .subsections_via_symbols (PR #87527)

[Fangrui Song via llvm-commits]

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@@ -927,6 +927,30 @@ void AsmPrinter::emitDebugValue(const MCExpr *Value, unsigned Size) const {
 
 void AsmPrinter::emitFunctionHeaderComment() {}
 
+void AsmPrinter::emitFunctionPrefix(
+    const SmallVector<const Constant *, 1> &Prefix) {
+  const Function &F = MF->getFunction();
+  if (!MAI->hasSubsectionsViaSymbols()) {
+    for (auto &C : Prefix) {
+      emitGlobalConstant(F.getParent()->getDataLayout(), C);
+    }
+    return;
+  }
+  // Preserving prefix data on platforms which use subsections-via-symbols
+  // is a bit tricky. Here we introduce a symbol for the prefix data
+  // and use the .alt_entry attribute to mark the function's real entry point
+  // as an alternative entry point to the prefix-data symbol.
+  MCSymbol *PrefixSym = OutContext.createLinkerPrivateTempSymbol();
+  OutStreamer->emitLabel(PrefixSym);
+
+  for (auto &C : Prefix) {
+    emitGlobalConstant(F.getParent()->getDataLayout(), C);
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MaskRay wrote:

omit braces https://llvm.org/docs/CodingStandards.html#don-t-use-braces-on-simple-single-statement-bodies-of-if-else-loop-statements

https://github.com/llvm/llvm-project/pull/87527