[RISC-V] Zicond increases static code size even with -Oz

[christian-herber-nxp]

In several workloads, I am seeing a code size expansion for Zicond.
This example of montgommery multiplication illustrates the scenario quite well:

https://godbolt.org/z/nKEv1dWra

The Zicond version is entirely branchless, which is great, but it comes at the expense of static code size, which should not happen at -Oz.

This function alone has several such cases, and they can be isolated, e.g.: https://godbolt.org/z/33P9szabs

uint64
simple (uint64 uhi, uint64 tlo, uint64 ulo)
{
  if (ulo < tlo)
    uhi = uhi + 1;

  return uhi;
}

Zicond uses 10 instructions, while no Zicond needs 8 to get the job done.

[llvmbot]

@llvm/issue-subscribers-backend-risc-v

Author: Christian Herber (christian-herber-nxp)

In several workloads, I am seeing a code size expansion for Zicond. This example of montgommery multiplication illustrates the scenario quite well:

https://godbolt.org/z/nKEv1dWra

The Zicond version is entirely branchless, which is great, but it comes at the expense of static code size, which should not happen at -Oz.

This function alone has several such cases, and they can be isolated, e.g.: https://godbolt.org/z/33P9szabs

uint64
simple (uint64 uhi, uint64 tlo, uint64 ulo)
{
  if (ulo < tlo)
    uhi = uhi + 1;

  return uhi;
}

Zicond uses 10 instructions, while no Zicond needs 8 to get the job done.

[preames]

I took a quick look and this isn't quite what it appears on the surface (to me). The 64 bit version of the simple example has the arithmetic expansion I'd expect. It's only the 32 bit (where the 64 bit pieces need legalized) that has the bad expansions. So this is some interaction of the zicond lowering and legalization. Will come back to this in the next day or so to take a deeper look.

[christian-herber-nxp]

@preames: any update on this one?

[preames]

@preames: any update on this one?

No, and not likely to in near future. Once I realized this was primarily a legalization issue, the overlap with near term work I'd planned dropped a lot. I've been preempted on to other activities anyways, so my chance of getting back to this soon are quite low.