[all-commits] [llvm/llvm-project] bb9449: [InstCombine] Fold @llvm.experimental.get.vector.l...

[Luke Lau via All-commits]

  Branch: refs/heads/main
  Home:   https://github.com/llvm/llvm-project
  Commit: bb9449d5bbd72441d8f95052ddfd29e2d29297d7
      https://github.com/llvm/llvm-project/commit/bb9449d5bbd72441d8f95052ddfd29e2d29297d7
  Author: Luke Lau <luke at igalia.com>
  Date:   2025-11-27 (Thu, 27 Nov 2025)

  Changed paths:
    M llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
    A llvm/test/Transforms/InstCombine/get_vector_length.ll

  Log Message:
  -----------
  [InstCombine] Fold @llvm.experimental.get.vector.length when cnt <= max_lanes (#169293)

On RISC-V, some loops that the loop vectorizer vectorizes pre-LTO may
turn out to have the exact trip count exposed after LTO, see #164762.

If the trip count is small enough we can fold away the
@llvm.experimental.get.vector.length intrinsic based on this corollary
from the LangRef:

> If %cnt is less than or equal to %max_lanes, the return value is equal
to %cnt.

This on its own doesn't remove the @llvm.experimental.get.vector.length
in #164762 since we also need to teach computeKnownBits about
@llvm.experimental.get.vector.length and the sub recurrence, but this PR
is a starting point.

I've added this in InstCombine rather than InstSimplify since we may
need to insert a truncation (@llvm.experimental.get.vector.length can
take an i64 %cnt argument, the result is always i32).

Note that there was something similar done in VPlan in #167647 for when
the loop vectorizer knows the trip count.



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