On Mon, 23 Oct 2023, Xi Ruoyao via Gcc-help wrote:
> GCC even uses branch instead of cmov for:
>
> int test(int x)
> {
> return x & 1 ? 3 * x + 1 : x / 2;
> }
>
> Should we create a ticket in bugzilla or is there some reason not to use
> cmov here?
There's a long-standing problem that RTL if-conversion passes do not emit
important information such as estimated costs and attempted transforms,
so it's impossible to say if it's "working as designed" without gdb'ing
the compiler. I suspect something is not working as it should on this
example.
In general:
For predictable branches, cmov can stitch computations into one long
dependency chain, making the resulting code latency-bound. For unpredictable
branches, cmov avoids the costs resulting from mispredictions.
The compiler cannot tell if a branch is going to be well-predictable or not.
My view is that compilers should offer __builtin_branchless_select so people
can express what they need. So far it didn't happen. LLVM offers
__builtin_unpredictable, which in my eyes is just an indirect way to request
branchless code.
Alexander
