[all-commits] [llvm/llvm-project] fa2a6d: [CodeGenPrepare][RISCV] Combine (X ^ Y) and (X == ...

[Ryan Buchner via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: fa2a6d68c6f35e478af3425760ae39e866049eee
      https://github.com/llvm/llvm-project/commit/fa2a6d68c6f35e478af3425760ae39e866049eee
  Author: Ryan Buchner <92571492+bababuck at users.noreply.github.com>
  Date:   2025-04-02 (Wed, 02 Apr 2025)

  Changed paths:
    M llvm/lib/CodeGen/CodeGenPrepare.cpp
    M llvm/lib/Target/RISCV/RISCVISelLowering.cpp
    M llvm/test/CodeGen/RISCV/select-constant-xor.ll

  Log Message:
  -----------
  [CodeGenPrepare][RISCV] Combine (X ^ Y) and (X == Y) where appropriate (#130922)

Fixes #130510.

In RISCV, modify the folding of (X ^ Y == 0) -> (X == Y) to account for
cases where the (X ^ Y) will be re-used.

If a constant is being used for the XOR before a branch, ensure that it
is small enough to fit within a 12-bit immediate field. Otherwise, the
equality check is more efficient than the check against 0, see the
following:
```
# %bb.0:
        lui     a1, 5
        addiw   a1, a1, 1365
        xor     a0, a0, a1
        beqz    a0, .LBB0_2
# %bb.1: 
        ret
.LBB0_2: 
```

```
# %bb.0:
        lui     a1, 5
        addiw   a1, a1, 1365
        beq    a0, a1, .LBB0_2
# %bb.1: 
        xor     a0, a0, a1
        ret
.LBB0_2: 
```

Similarly, if the XOR is between 1 and a size one integer, we should
still fold away the XOR since that comparison can be optimized as a
comparison against 0.
```
# %bb.0:
        slt a0, a0, a1
        xor  a0, a0, 1
        beqz    a0, .LBB0_2
# %bb.1: 
        ret
.LBB0_2: 
```

```
# %bb.0:
        slt a0, a0, a1
        bnez    a0, .LBB0_2
# %bb.1: 
        xor  a0, a0, 1
        ret
.LBB0_2: 
```

One question about my code is that I used a hard-coded value for the
width of a RISCV ALU immediate. Do you know of a way that I can gather
this from the `context`, I was unable to devise one.



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