[PATCH] D51026: [CodeGen] Implicitly set stackrealign on the main function, if custom stack alignment is used

[Martin Storsjö via Phabricator via cfe-commits]

mstorsjo created this revision.
mstorsjo added reviewers: craig.topper, erichkeane, rnk.

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.


Repository:
  rC Clang

https://reviews.llvm.org/D51026

Files:
  lib/CodeGen/CodeGenFunction.cpp
  test/CodeGen/stackrealign-main.c


Index: test/CodeGen/stackrealign-main.c
===================================================================
--- /dev/null
+++ test/CodeGen/stackrealign-main.c
@@ -0,0 +1,19 @@
+// 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"{{.*}} }
Index: lib/CodeGen/CodeGenFunction.cpp
===================================================================
--- lib/CodeGen/CodeGenFunction.cpp
+++ lib/CodeGen/CodeGenFunction.cpp
@@ -981,6 +981,13 @@
       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


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