Skip to content

[LV] Restrict scalable vectorization to targets with power-of-2 vscale #183065

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Closed
lukel97 wants to merge 1 commit into from
Closed

[LV] Restrict scalable vectorization to targets with power-of-2 vscale #183065

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
3 changes: 0 additions & 3 deletions llvm/lib/Target/RISCV/RISCVISelLowering.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -25717,9 +25717,6 @@ bool RISCVTargetLowering::isVScaleKnownToBeAPowerOfTwo() const {
// We define vscale to be VLEN/RVVBitsPerBlock. VLEN is always a power
// of two >= 64, and RVVBitsPerBlock is 64. Thus, vscale must be
// a power of two as well.
// FIXME: This doesn't work for zve32, but that's already broken
// elsewhere for the same reason.
assert(Subtarget.getRealMinVLen() >= 64 && "zve32* unsupported");
Copy link
Copy Markdown
Contributor Author

@lukel97 lukel97 Feb 24, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

llvm/test/Transforms/LoopVectorize/RISCV/zvl32b.ll triggers this assert because we query isVScaleKnownToBeAPowerOfTwo earlier.

static_assert(RISCV::RVVBitsPerBlock == 64,
"RVVBitsPerBlock changed, audit needed");
return true;
Expand Down
10 changes: 7 additions & 3 deletions llvm/lib/Transforms/Vectorize/LoopVectorize.cpp
View file
Open in desktop
Original file line number Diff line number Diff line change
Expand Up @@ -296,9 +296,9 @@ cl::opt<unsigned> llvm::ForceTargetInstructionCost(

static cl::opt<bool> ForceTargetSupportsScalableVectors(
"force-target-supports-scalable-vectors", cl::init(false), cl::Hidden,
cl::desc(
"Pretend that scalable vectors are supported, even if the target does "
"not support them. This flag should only be used for testing."));
cl::desc("Pretend that scalable vectors are supported and vscale is a "
"power of two, even if the target does "
Copy link
Copy Markdown
Contributor Author

@lukel97 lukel97 Feb 24, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

This adjusts the wording so that we can assume power-of-2 vscale in the target independent tests

Copy link
Copy Markdown
Contributor

@artagnon artagnon Feb 25, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I'd probably revert this specific hunk, since the LangRef change has landed.

"not support them. This flag should only be used for testing."));

static cl::opt<unsigned> SmallLoopCost(
"small-loop-cost", cl::init(20), cl::Hidden,
Expand Down Expand Up @@ -3387,6 +3387,10 @@ bool LoopVectorizationCostModel::isScalableVectorizationAllowed() {
if (!TTI.supportsScalableVectors() && !ForceTargetSupportsScalableVectors)
return false;

if (!TTI.isVScaleKnownToBeAPowerOfTwo() &&
Copy link
Copy Markdown
Contributor

@david-arm david-arm Feb 25, 2026

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I think it probably makes sense to now just remove this hook completely, right?

Copy link
Copy Markdown
Collaborator

@paulwalker-arm paulwalker-arm Feb 25, 2026 •
edited
Loading

Choose a reason for hiding this comment

The reason will be displayed to describe this comment to others. Learn more.

I am working on PR to do this. I just wanted to land the LangRef change first to unblock other related work.

#183292

!ForceTargetSupportsScalableVectors)
return false;

if (Hints->isScalableVectorizationDisabled()) {
reportVectorizationInfo("Scalable vectorization is explicitly disabled",
"ScalableVectorizationDisabled", ORE, TheLoop);
Expand Down