[all-commits] [llvm/llvm-project] 2e21bb: [RISCV][ISelLowering] Use Zicond for FP selects on...

[fennecJ via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 2e21bb815d527ebbe4d53f0396d1e40aae9e2146
      https://github.com/llvm/llvm-project/commit/2e21bb815d527ebbe4d53f0396d1e40aae9e2146
  Author: fennecJ <hwahwa649 at gmail.com>
  Date:   2025-12-01 (Mon, 01 Dec 2025)

  Changed paths:
    M llvm/lib/Target/RISCV/RISCVISelLowering.cpp
    A llvm/test/CodeGen/RISCV/zicond-fp-select-zfinx.ll

  Log Message:
  -----------
  [RISCV][ISelLowering] Use Zicond for FP selects on Zfinx/Zdinx (#169299)

### Summary

This patch let RISCVTargetLowering::lowerSELECT to lower some
floating-point select operations through an integer zicond select when:
* Zicond is available, and 
* FP values live in GPRs (Zfinx/Zdinx), and
* Select condition is an integer type.

In that scenario there is no extra cost for GPR <-> "FP GPR" moves, so
we can implement FP selects with a CZERO-based sequence instead of a
branch.

For example, for  
```c
float foo(int cond, float x) {
    return (cond != 0) ? x : 0.0f;
}
```
the current lowering produces:
```asm
foo:
  mv    a2, a0
  li    a0, 0
  beqz  a2, .LBB0_2
.LBB0_1:
  mv    a0, a1
.LBB0_2:
  ret
```

With this patch, when targeting rv64ima_zicond_zfinx we instead get:  

```asm
foo:
  czero.nez  a2, zero, a0
  czero.eqz  a0, a1, a0
  or         a0, a2, a0
  ret
```


The existing branch-based lowering is preserved for:
* targets without Zicond
* targets where FP registers are separate (+f, +d without zfinx/zdinx)

### Testing

Adds llvm/test/CodeGen/RISCV/zicond-fp-select-zfinx.ll to cover:
* RV64 Zfinx/Zicond vs Zfinx without Zicond
* RV64 Zdinx/Zicond vs Zdinx without Zicond
* RV32 Zfinx/Zicond vs Zfinx without Zicond

Also adds baseline RV32F/RV64F/RV64D cases to ensure we still use
branches when FP registers are separate.

The tests check that:
* With Zicond + Zfinx/Zdinx, FP select lowers to a CZERO+OR sequence
with no conditional branches.
* Without Zicond (or without Zfinx/Zdinx), we still get branch-based
code and no czero.* instructions.



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