https://gcc.gnu.org/bugzilla/show_bug.cgi?id=100377
Bug ID: 100377
Summary: needless stack adjustment when passing struct in
register
Product: gcc
Version: 12.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: luc.vanoostenryck at gmail dot com
Target Milestone: ---
Created attachment 50726
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50726&action=edit
testcases
When compiling with optimization for example -O2), the following code:
struct sb {
signed char a;
char b;
short y[3];
};
struct ub {
unsigned char a;
char b;
short y[3];
};
int fsb(struct sb s) { return s.a; }
int fub(struct ub s) { return s.a; }
produces the following assembly code on arm64:
fsb:
sub sp, sp, #16
sxtb w0, w0
add sp, sp, 16
ret
fub:
sub sp, sp, #16
and w0, w0, 255
add sp, sp, 16
ret
the following on mips64:
fsb:
daddiu $sp,$sp,-16
dsll $2,$4,56
dsra $2,$2,56
j $31
daddiu $sp,$sp,16
fub:
daddiu $sp,$sp,-16
andi $2,$4,0xff
j $31
daddiu $sp,$sp,16
the following on riscv64:
fsb:
addi sp,sp,-16
slli a0,a0,24
srai a0,a0,24
addi sp,sp,16
jr ra
fub:
addi sp,sp,-16
andi a0,a0,0xff
addi sp,sp,16
jr ra
OTOH, things seems OK on ppc64:
fsb:
extsb 3,3
blr
fub:
rlwinm 3,3,0,0xff
blr
and x86_64:
fsb:
movsx eax, dil
ret
fub:
movzx eax, dil
ret
Similar problems happen on 32-bit platforms too.
For example on arm32, the following code:
struct ub32 {
unsigned char a;
char b;
short y[1];
};
int fub32(struct ub32 s) { return s.a; }
produces:
fub32:
sub sp, sp, #8
uxtb r0, r0
add sp, sp, #8
bx lr
All these seem to happen on all versions.
See https://gcc.godbolt.org/z/x9zc1EnYn
Note: similar PRs exist but reported for x86_64 only
