[llvm-dev] Optimizing assembly generated for tail call

[Joerg Sonnenberger via llvm-dev]

On Tue, Oct 06, 2020 at 02:51:46AM -0700, Haoran Xu via llvm-dev wrote:
> Hello,
> 
> I recently found that LLVM generates sub-optimal assembly for a tail call
> optimization case. Below is an example (https://godbolt.org/z/ao15xE):
> 
> > void g1();
> > void g2();
> > void f(bool v) {
> >    if (v) {
> >     g1();
> >    } else {
> >      g2();
> >    }
> > }
> >
> The assembly generated is as follow:
> 
> > f(bool): # @f(bool)
> > testb %dil, %dil
> > je .LBB0_2
> > jmp g1() # TAILCALL
> > .LBB0_2:
> > jmp g2() # TAILCALL
> >
> However, in this specific case (where no function epilogue is needed), one
> can actually change 'je .LBB0_2' to 'je g2()' directly, thus saving a jump.
> Is there any way I could instruct LLVM to do this? For my use case, it is
> acceptable to do this at any level (C++ level /IR level /MachineInst level
> is all fine).

That translation doesn't normally work as conditional jumps have a much
more restricted range.

Joerg