stack-overalign.ll   [plain text]

; RUN: llvm-as < %s | opt -instcombine | llvm-dis | grep {align 32} | count 1

; It's tempting to have an instcombine in which the src pointer of a
; memcpy is aligned up to the alignment of the destination, however
; there are pitfalls. If the src is an alloca, aligning it beyond what
; the target's stack pointer is aligned at will require dynamic
; stack realignment, which can require functions that don't otherwise
; need a frame pointer to need one.
;
; Abstaining from this transform is not the only way to approach this
; issue. Some late phase could be smart enough to reduce alloca
; alignments when they are greater than they need to be. Or, codegen
; could do dynamic alignment for just the one alloca, and leave the
; main stack pointer at its standard alignment.

@dst = global [1024 x i8] zeroinitializer, align 32

define void @foo() nounwind {
entry:
	%src = alloca [1024 x i8], align 1
	%src1 = getelementptr [1024 x i8]* %src, i32 0, i32 0
	call void @llvm.memcpy.i32(i8* getelementptr ([1024 x i8]* @dst, i32 0, i32 0), i8* %src1, i32 1024, i32 1)
	call void @frob(i8* %src1) nounwind
	ret void
}

declare void @llvm.memcpy.i32(i8* nocapture, i8* nocapture, i32, i32) nounwind

declare void @frob(i8*)