On 7/14/25 8:52 AM, Richard Henderson wrote:
On 7/14/25 09:41, Pierrick Bouvier wrote:
Indeed, clang does not fold the condition "value && kvm_enabled() && !
kvm_arm_sve_supported()". Looks like a missing case.
This code compiles with gcc -O0, but not clang -O0.
extern int f(void);
int main(int argc) {
if (argc && 0)
f();
}
As folding is not guaranteed by C standard, I'm not sure it's really possible
to file a
bug. However, since we rely on this behaviour in other parts, maybe it would be
better to
rewrite the condition on our side.
It's probably worth filing a missed-optimization type bug, if that's available
in clang's
reporting system.
Sure, I'll post a bug report on clang repository.
With my compiler hat on, I suspect that GCC generates IR like
if (argc) {
if (0) {
f();
}
}
in order to get the short-circuting part of && correct, which Just So Happens
to fold away
exactly as we wish.
I'm not sure how clang expands the expression such that (x && 0) doesn't fold
away, but (0
&& x) does, as evidenced by
For gcc, the simple GIMPLE tree is identical for both.
int main (int argc)
{
int D.2775;
{
if (0 != 0) goto <D.2773>; else goto <D.2774>;
<D.2773>:
f ();
<D.2774>:
}
D.2775 = 0;
return D.2775;
}
This is the LLVM IR difference between "(0 && argc)" and "(argc && 0)".
; Function Attrs: noinline nounwind optnone uwtable
define dso_local i32 @main(i32 noundef %0) #0 {
%2 = alloca i32, align 4
- %3 = alloca i32, align 4
- store i32 0, ptr %2, align 4
- store i32 %0, ptr %3, align 4
- %4 = load i32, ptr %3, align 4
- %5 = icmp ne i32 %4, 0
- br i1 %5, label %6, label %9
-
-6: ; preds = %1
- br i1 false, label %7, label %9
-
-7: ; preds = %6
- %8 = call i32 @f()
- br label %9
-
-9: ; preds = %7, %6, %1
- %10 = load i32, ptr %2, align 4
- ret i32 %10
+ store i32 %0, ptr %2, align 4
+ ret i32 0
}
-declare i32 @f() #1
-
We can see it elides completely the if as expected, given the right
order for expression.
On a side note, the first operand still needs to be evaluated if it has
a side effect (especially a function call), before ensuring the shortcut
properly applies.
+ if (kvm_enabled() && !kvm_arm_sve_supported()) {
r~
