Re: [PATCH] target/arm: Provide always-false kvm_arm_*_supported() stubs for usermode

[Richard Henderson <richard.henderson@linaro.org>]

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.

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

> +        if (kvm_enabled() && !kvm_arm_sve_supported()) { 


r~