[CodeGen] Implicitly set stackrealign on the main function, if custom stack alignment is used

[mstorsjo]

Summary

If using a custom stack alignment, one is expected to make sure that all callers provide such alignment, or realign the stack in all entry points (and callbacks).

Despite this, the compiler can assume that the main function will need realignment in these cases, since the startup routines calling the main function most probably won't provide the custom alignment.

This matches what GCC does in similar cases; if compiling with -mincoming-stack-boundary=X -mpreferred-stack-boundary=X, GCC normally assumes such alignment on entry to a function, but specifically for the main function still does realignment.

[erichkeane]

lib/CodeGen/CodeGenFunction.cpp

989

Is there not an attribute name for this already in LLVM? I guess I'm otherwise fine with this, but would want @craig.topper or @rnk to confirm that we're Ok just sending this string as an attribute.

[mstorsjo]

lib/CodeGen/CodeGenFunction.cpp

989

I don't see one in llvm/include/llvm/IR/Attributes.td at least, and all other occurrances in clang just use the plain string.

[rnk]

lgtm

lib/CodeGen/CodeGenFunction.cpp

989

Yep, we use the string stackrealign attribute.

lib/CodeGen/CodeGenFunction.cpp

#undef SANITIZER
   // If we're in C++ mode and the function name is "main", it is guaranteed
   // to be norecurse by the standard (3.6.1.3 "The function main shall not be
   // used within a program").
   if (getLangOpts().CPlusPlus)
     if (const FunctionDecl *FD = dyn_cast_or_null<FunctionDecl>(D))
       if (FD->isMain())
         Fn->addFnAttr(llvm::Attribute::NoRecurse);
 
+  // If a custom alignment is used, force realigning to this alignment on
+  // any main function which certainly will need it.
+  if (const FunctionDecl *FD = dyn_cast_or_null<FunctionDecl>(D))
+    if ((FD->isMain() || FD->isMSVCRTEntryPoint()) &&
+        CGM.getCodeGenOpts().StackAlignment)
+      Fn->addFnAttr("stackrealign");
+
   llvm::BasicBlock *EntryBB = createBasicBlock("entry", CurFn);
 
   // Create a marker to make it easy to insert allocas into the entryblock
   // later.  Don't create this with the builder, because we don't want it
   // folded.
   llvm::Value *Undef = llvm::UndefValue::get(Int32Ty);
   AllocaInsertPt = new llvm::BitCastInst(Undef, Int32Ty, "allocapt", EntryBB);
 

test/CodeGen/stackrealign-main.c

+// RUN: %clang_cc1 -triple i386-unknown-unknown -emit-llvm -o - -mstack-alignment=64 %s | FileCheck %s
+
+// CHECK-LABEL: define void @other()
+// CHECK: [[OTHER:#[0-9]+]]
+// CHECK: {
+void other(void) {}
+
+// CHECK-LABEL: define i32 @main(
+// CHECK: [[MAIN:#[0-9]+]]
+// CHECK: {
+int main(int argc, char **argv) {
+  other();
+  return 0;
+}
+
+// CHECK: attributes [[OTHER]] = { noinline nounwind optnone
+// CHECK-NOT: "stackrealign"
+// CHECK: }
+// CHECK: attributes [[MAIN]] = { noinline nounwind optnone {{.*}}"stackrealign"{{.*}} }