https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108539
Bug ID: 108539
Summary: Wrong register usage for -m16 -masm=intel -march=i386
on asm volatile
Product: gcc
Version: 12.1.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: c++
Assignee: unassigned at gcc dot gnu.org
Reporter: arheik at dnainternet dot net
Target Milestone: ---
Consider the following code (test.cpp):
void test(unsigned char& a, unsigned char& b)
{
unsigned char _a, _b;
asm volatile(
"mov dx, 0\n"
"mov bx, 0\n"
: "=dl" (_a), "=dh" (_b)
:
: "bx"
);
a = _a;
b = _b;
}
void main(void)
{
unsigned char a, b;
test(a, b);
}
Compiled with:
g++ -m16 -masm=intel -march=i386 -ffreestanding -fno-inline -mregparm=3
-fno-dwarf2-cfi-asm -fno-asynchronous-unwind-tables -fno-ident -O2 test.cpp -o
- -S
This generates for test():
_Z4testRhS_:
.LFB0:
push esi
.LCFI0:
push ebx
.LCFI1:
mov ecx, edx
#APP
# 4 "test.cpp" 1
mov dx, 0
mov bx, 0
# 0 "" 2
#NO_APP
mov ebx, esi
mov BYTE PTR [eax], bl
mov BYTE PTR [ecx], dl
pop ebx
.LCFI2:
pop esi
.LCFI3:
ret
In which bl usage is clearly wrong (dh is missing) and ebx/eax/esi usage looks
suspect.
Compling with -mregparm=0 produces for test():
_Z4testRhS_:
.LFB0:
push ebx
.LCFI0:
#APP
# 4 "test.cpp" 1
mov dx, 0
mov bx, 0
# 0 "" 2
#NO_APP
mov eax, DWORD PTR 8[esp]
mov BYTE PTR [eax], cl
mov eax, DWORD PTR 12[esp]
mov BYTE PTR [eax], dl
pop ebx
.LCFI1:
ret
Which should use dh instead of cl.
