https://gcc.gnu.org/bugzilla/show_bug.cgi?id=128117
Bug ID: 128117
Summary: [17 Regression] wrong code at -O2: code that is not a
multiplication is turned into a high-part multiply
Product: gcc
Version: 17.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: yuki.tanaka at defaultcase dot de
Target Milestone: ---
__attribute__((noipa)) unsigned long long a(unsigned long long x, unsigned long
long y)
{
unsigned long long xl = x & 0xffffffff, xh = x >> 32;
unsigned long long yl = y & 0xffffffff, yh = y >> 32;
unsigned long long t0 = xl * yl;
unsigned long long t1 = xh * yl;
unsigned long long t2 = xl * yh;
unsigned long long t3 = xh * yh;
unsigned long long s = (t1 & 0xffffffff) + t2 + (t0 >> 32);
return t3 + (t2 >> 32) + (s >> 32);
}
__attribute__((noipa)) unsigned long long b(unsigned long long x, unsigned long
long y)
{
unsigned long long xl = x & 0xffffffff, xh = x >> 32;
unsigned long long yl = y & 0xffffffff, yh = y >> 32;
unsigned long long t0 = xl * yl;
unsigned long long t1 = xh * yl;
unsigned long long t2 = xl * yh;
unsigned long long t3 = xh * yh;
unsigned long long u0 = (t0 >> 32) + t1;
unsigned long long u1 = (u0 & 0xffffffff) + t2;
unsigned long long u2 = (t0 >> 32) + t2;
return t3 + (u2 >> 32) + (u1 >> 32);
}
int main ()
{
if (a(0xffffffff00000000ULL, 0xffffffffULL) != 0)
__builtin_abort ();
if (b(0xffffffff00000000ULL, 0xffffffffULL) != 0)
__builtin_abort ();
return 0;
}
At -O2 GCC 17 aborts, GCC 16.1 does not. GCC 17 with -O1 or -O2
-fno-tree-forwprop are fine.
Compiler Explorer, -O2 (GCC 16.1 vs. GCC 17):
x86-64 GCC 17.0.0 20261023, -O2:
"a":
mov rax, rdi
mul rsi
mov rax, rdx
ret
"b":
mov rax, rdi
mul rsi
mov rax, rdx
ret
Possibly related to r17-3440-g91cca3ac45e7 ("forwprop: Add long-multiply ladder
variants", PR107090).
