[all-commits] [llvm/llvm-project] 84c849: [InstCombine] Combine interleaved recurrences. (#1...

[Ricardo Jesus via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: 84c849e85b0f8ab2d8bbeb2d9c46349f019f4e8e
      https://github.com/llvm/llvm-project/commit/84c849e85b0f8ab2d8bbeb2d9c46349f019f4e8e
  Author: Ricardo Jesus <rjj at nvidia.com>
  Date:   2025-07-01 (Tue, 01 Jul 2025)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineInternal.h
    M llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
    A llvm/test/Transforms/InstCombine/binop-recurrence.ll

  Log Message:
  -----------
  [InstCombine] Combine interleaved recurrences. (#143878)

Combine sequences such as:
```llvm
  %pn1 = phi [init1, %BB1], [%op1, %BB2]
  %pn2 = phi [init2, %BB1], [%op2, %BB2]
  %op1 = binop %pn1, constant1
  %op2 = binop %pn2, constant2
  %rdx = binop %op1, %op2
```
Into:
```llvm
  %phi_combined = phi [init_combined, %BB1], [%op_combined, %BB2]
  %rdx_combined = binop %phi_combined, constant_combined
```

This allows us to simplify interleaved reductions, for example as
introduced by the loop vectorizer.

The anecdotal example for this is the loop below:
```c
float foo() {
  float q = 1.f;
  for (int i = 0; i < 1000; ++i)
    q *= .99f;
  return q;
}
```
Which currently gets lowered explicitly such as (on AArch64,
interleaved by four):
```gas
.LBB0_1:
  fmul    v0.4s, v0.4s, v1.4s
  fmul    v2.4s, v2.4s, v1.4s
  fmul    v3.4s, v3.4s, v1.4s
  fmul    v4.4s, v4.4s, v1.4s
  subs    w8, w8, #32
  b.ne    .LBB0_1
```
But with this patch lowers trivially:
```gas
foo:
  mov     w8, #5028
  movk    w8, #14389, lsl #16
  fmov    s0, w8
  ret
```



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