https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92712
Bug ID: 92712
Summary: Performance regression with assumed values
Product: gcc
Version: 9.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: rtl-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: mike.k at digitalcarbide dot com
Target Milestone: ---
The following code generates progressively worse code from GCC 7.5 to GCC 8.3
to GCC 9.1 (and trunk):
static void func_base(int t, const int v) {
int x = 0;
for (int i = 0; i < t; ++i) {
x += v;
}
volatile int d = x;
}
void func_default(int t, const int v) {
func_base(t, v);
}
void func_assumed(int t, const int v) {
if (t < 0) __builtin_unreachable();
func_base(t, v);
}
On GCC 7.5 (-O2):
func_default(int, int):
test edi, edi
jle .L3
imul edi, esi
mov DWORD PTR [rsp-4], edi
ret
.L3:
xor edi, edi
mov DWORD PTR [rsp-4], edi
ret
func_assumed(int, int):
imul edi, esi
mov DWORD PTR [rsp-4], edi
ret
On GCC 8.3 (-O2):
func_default(int, int):
test edi, edi
jle .L3
imul edi, esi
mov DWORD PTR [rsp-4], edi
ret
.L3:
xor edi, edi
mov DWORD PTR [rsp-4], edi
ret
func_assumed(int, int):
test edi, edi
je .L6
imul edi, esi
.L6:
mov DWORD PTR [rsp-4], edi
ret
On GCC 9.1 and trunk (-O2):
func_default(int, int):
test edi, edi
jle .L3
sub edi, 1
imul edi, esi
add esi, edi
mov DWORD PTR [rsp-4], esi
ret
.L3:
xor esi, esi
mov DWORD PTR [rsp-4], esi
ret
func_assumed(int, int):
test edi, edi
je .L6
sub edi, 1
imul edi, esi
add edi, esi
.L6:
mov DWORD PTR [rsp-4], edi
ret
This occurs regardless of if `func_base` is allowed to inline, or if it is
manually inlined.
It does not occur in LLVM-Clang or in Microsoft Visual C++.
