[CodeGen] Provide an appropriate alignment for dynamic allocas

[majnemer]

Summary

GCC documents builtin_alloca as aligning the storage to at least
BIGGEST_ALIGNMENT__.

MSVC documents essentially the same for the x64 ABI:
https://msdn.microsoft.com/en-us/library/x9sx5da1.aspx

The 32-bit ABI follows the same rule: it emits a call to _alloca_probe_16

[rnk]

lgtm

[efriedma]

This is probably going to lead to someone complaining about clang realigning the stack on 32-bit Windows; are your sure we're choosing the right alignment there?

[rnk]

In D24378#538430, @efriedma wrote:

This is probably going to lead to someone complaining about clang realigning the stack on 32-bit Windows; are your sure we're choosing the right alignment there?

I checked, and MSVC emits a call to __alloca_probe_16 for this code:

extern "C" void *_alloca(size_t);
#pragma intrinsic (_alloca)
void g(void*);
void f(int n) {
  void *p = _alloca(n);
  g(p);
}

So, they do realign the stack. We don't really need to realign the whole stack frame for highly aligned dynamic allocas, though. There's no reason we can't overallocate from the stack and realign the returned pointer. I'm not sure if we do that, though...

[efriedma]

Huh... then I guess this is fine.

[efriedma]

Ping

cfe/trunk/lib/CodeGen/CGBuiltin.cpp

case Builtin::BI__builtin_fpclassify: {
     Builder.SetInsertPoint(End);
     return RValue::get(Result);
   }
 
   case Builtin::BIalloca:
   case Builtin::BI_alloca:
   case Builtin::BI__builtin_alloca: {
     Value *Size = EmitScalarExpr(E->getArg(0));
-    return RValue::get(Builder.CreateAlloca(Builder.getInt8Ty(), Size));
+    const TargetInfo &TI = getContext().getTargetInfo();
+    // The alignment of the alloca should correspond to __BIGGEST_ALIGNMENT__.
+    unsigned SuitableAlignmentInBytes =
+        TI.getSuitableAlign() / TI.getCharWidth();
+    AllocaInst *AI = Builder.CreateAlloca(Builder.getInt8Ty(), Size);
+    AI->setAlignment(SuitableAlignmentInBytes);
+    return RValue::get(AI);
   }
   case Builtin::BIbzero:
   case Builtin::BI__builtin_bzero: {
     Address Dest = EmitPointerWithAlignment(E->getArg(0));
     Value *SizeVal = EmitScalarExpr(E->getArg(1));
     EmitNonNullArgCheck(RValue::get(Dest.getPointer()), E->getArg(0)->getType(),
                         E->getArg(0)->getExprLoc(), FD, 0);
     Builder.CreateMemSet(Dest, Builder.getInt8(0), SizeVal, false);

cfe/trunk/test/CodeGen/builtins-ms.c

 // RUN: %clang_cc1 %s -emit-llvm -o - -fms-extensions -triple i686-pc-win32 | FileCheck %s
 
 // CHECK-LABEL: define void @test_alloca
 void capture(void *);
 void test_alloca(int n) {
   capture(_alloca(n));
-  // CHECK: %[[arg:.*]] = alloca i8, i32 %
+  // CHECK: %[[arg:.*]] = alloca i8, i32 %{{.*}}, align 16
   // CHECK: call void @capture(i8* %[[arg]])
 }