From d39d53b0d1a55370a46c6f121e11b5c43a0396db Mon Sep 17 00:00:00 2001
From: Martin Storsjo <martin@martin.st>
Date: Tue, 21 Aug 2018 20:41:17 +0000
Subject: [PATCH] [CodeGen] Implicitly set stackrealign on the main function,
 if custom stack alignment is used

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.

Differential Revision: https://reviews.llvm.org/D51026

llvm-svn: 340334
---
 clang/lib/CodeGen/CodeGenFunction.cpp  |  7 +++++++
 clang/test/CodeGen/stackrealign-main.c | 19 +++++++++++++++++++
 2 files changed, 26 insertions(+)
 create mode 100644 clang/test/CodeGen/stackrealign-main.c

diff --git a/clang/lib/CodeGen/CodeGenFunction.cpp b/clang/lib/CodeGen/CodeGenFunction.cpp
index 030101d2f4ed..d3b1bd77dc43 100644
--- a/clang/lib/CodeGen/CodeGenFunction.cpp
+++ b/clang/lib/CodeGen/CodeGenFunction.cpp
@@ -979,6 +979,13 @@ void CodeGenFunction::StartFunction(GlobalDecl GD,
       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
diff --git a/clang/test/CodeGen/stackrealign-main.c b/clang/test/CodeGen/stackrealign-main.c
new file mode 100644
index 000000000000..10b21bf548db
--- /dev/null
+++ b/clang/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"{{.*}} }
-- 
GitLab