If a parameter is passed on stack, caller is responsible to align
the stack frame to at least the alignment of the parameter. But gcc
will align the stack even when it has been aligned already by caller.
bash-3.2$ cat 1.i
typedef float __m128 __attribute__ ((__vector_size__ (16), __may_alias__));
int
__attribute__ ((noinline))
foo(__m128 x, int size, ...)
{
volatile char * ptr=__builtin_alloca(size);
volatile int __attribute((aligned(16))) xxx;
xxx = 2;
ptr [1]= 30;
return xxx;
}
int
bar ()
{
__m128 x = { 1.0 };
foo (x, 30);
return 0;
}
bash-3.2$ /export/build/gnu/gcc-avx/build-x86_64-linux/gcc/xgcc
-B/export/build/gnu/gcc-stack/build-x86_64-linux/gcc/ -msse2 -m32
-mpreferred-stack-boundary=2 -S -o 1.s 1.i
bash-3.2$ cat 1.s
.file "1.i"
.text
.globl foo
.type foo, @function
foo:
leal 4(%esp), %ecx
andl $-16, %esp
pushl -4(%ecx)
pushl %ebp
movl %esp, %ebp
pushl %ecx
subl $36, %esp
movl 16(%ecx), %eax
addl $15, %eax
addl $3, %eax
shrl $2, %eax
sall $2, %eax
subl %eax, %esp
movl %esp, -28(%ebp)
movl -28(%ebp), %eax
addl $15, %eax
shrl $4, %eax
sall $4, %eax
movl %eax, -28(%ebp)
movl -28(%ebp), %eax
movl %eax, -12(%ebp)
movl $2, -24(%ebp)
movl -12(%ebp), %eax
addl $1, %eax
movb $30, (%eax)
movl -24(%ebp), %eax
movl -4(%ebp), %ecx
leave
leal -4(%ecx), %esp
ret
.size foo, .-foo
.globl bar
.type bar, @function
bar:
pushl %ebp
movl %esp, %ebp
andl $-16, %esp
subl $48, %esp
movss .LC0, %xmm0
movlps %xmm0, 32(%esp)
movhps %xmm0, 40(%esp)
movl $30, 16(%esp)
xorps %xmm0, %xmm0
movlps 32(%esp), %xmm0
movhps 40(%esp), %xmm0
movlps %xmm0, (%esp)
movhps %xmm0, 8(%esp)
call foo
movl $0, %eax
leave
ret
.size bar, .-bar
.section .rodata
.align 16
.LC0:
.long 1065353216
.long 0
.long 0
.long 0
.ident "GCC: (GNU) 4.4.0 20080802 (experimental) [stack revision
138551]"
.section .note.GNU-stack,"",@progbits
bash-3.2$
--
Summary: No need to align stack when incoming stack is aligned
Product: gcc
Version: 4.4.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: middle-end
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: hjl dot tools at gmail dot com
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37009
