xref: /llvm-project/llvm/test/Transforms/InstCombine/stack-overalign.ll (revision 4ab40eca080965c65802710e39adbb78c4ce7bde)
1; RUN: opt < %s -passes=instcombine -S | grep "align 32" | count 2
2
3; It's tempting to have an instcombine in which the src pointer of a
4; memcpy is aligned up to the alignment of the destination, however
5; there are pitfalls. If the src is an alloca, aligning it beyond what
6; the target's stack pointer is aligned at will require dynamic
7; stack realignment, which can require functions that don't otherwise
8; need a frame pointer to need one.
9;
10; Abstaining from this transform is not the only way to approach this
11; issue. Some late phase could be smart enough to reduce alloca
12; alignments when they are greater than they need to be. Or, codegen
13; could do dynamic alignment for just the one alloca, and leave the
14; main stack pointer at its standard alignment.
15;
16
17
18@dst = global [1024 x i8] zeroinitializer, align 32
19
20define void @foo() nounwind {
21entry:
22  %src = alloca [1024 x i8], align 64
23  call void @llvm.memcpy.p0.p0.i32(ptr align 32 @dst, ptr align 32 %src, i32 1024, i1 false)
24  call void @frob(ptr %src) nounwind
25  ret void
26}
27
28declare void @frob(ptr)
29
30declare void @llvm.memcpy.p0.p0.i32(ptr nocapture, ptr nocapture, i32, i1) nounwind
31