[llvm] [InstCombine] Fold @llvm.experimental.get.vector.length when cnt <= max_lanes (PR #169293)
Luke Lau via llvm-commits
llvm-commits at lists.llvm.org
Wed Nov 26 22:40:31 PST 2025
https://github.com/lukel97 updated https://github.com/llvm/llvm-project/pull/169293
>From c61d4f26dac28b98622d395f61e90ec1f4a87bf2 Mon Sep 17 00:00:00 2001
From: Luke Lau <luke at igalia.com>
Date: Mon, 24 Nov 2025 16:09:35 +0800
Subject: [PATCH 1/4] Precommit tests
---
.../InstCombine/get_vector_length.ll | 83 +++++++++++++++++++
1 file changed, 83 insertions(+)
create mode 100644 llvm/test/Transforms/InstCombine/get_vector_length.ll
diff --git a/llvm/test/Transforms/InstCombine/get_vector_length.ll b/llvm/test/Transforms/InstCombine/get_vector_length.ll
new file mode 100644
index 0000000000000..2925d6f556988
--- /dev/null
+++ b/llvm/test/Transforms/InstCombine/get_vector_length.ll
@@ -0,0 +1,83 @@
+; NOTE: Assertions have been autogenerated by utils/update_test_checks.py UTC_ARGS: --version 6
+; RUN: opt < %s -passes=instcombine,verify -S | FileCheck %s
+
+define i32 @cnt_known_lt() {
+; CHECK-LABEL: define i32 @cnt_known_lt() {
+; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 1, i32 2, i1 false)
+; CHECK-NEXT: ret i32 [[X]]
+;
+ %x = call i32 @llvm.experimental.get.vector.length(i32 1, i32 2, i1 false)
+ ret i32 %x
+}
+
+define i32 @cnt_not_known_lt() {
+; CHECK-LABEL: define i32 @cnt_not_known_lt() {
+; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 2, i32 1, i1 false)
+; CHECK-NEXT: ret i32 [[X]]
+;
+ %x = call i32 @llvm.experimental.get.vector.length(i32 2, i32 1, i1 false)
+ ret i32 %x
+}
+
+define i32 @cnt_known_lt_scalable() vscale_range(2, 4) {
+; CHECK-LABEL: define i32 @cnt_known_lt_scalable(
+; CHECK-SAME: ) #[[ATTR0:[0-9]+]] {
+; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 2, i32 1, i1 true)
+; CHECK-NEXT: ret i32 [[X]]
+;
+ %x = call i32 @llvm.experimental.get.vector.length(i32 2, i32 1, i1 true)
+ ret i32 %x
+}
+
+define i32 @cnt_not_known_lt_scalable() {
+; CHECK-LABEL: define i32 @cnt_not_known_lt_scalable() {
+; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 2, i32 1, i1 true)
+; CHECK-NEXT: ret i32 [[X]]
+;
+ %x = call i32 @llvm.experimental.get.vector.length(i32 2, i32 1, i1 true)
+ ret i32 %x
+}
+
+define i32 @cnt_known_lt_runtime(i32 %x) {
+; CHECK-LABEL: define i32 @cnt_known_lt_runtime(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT: [[ICMP:%.*]] = icmp ult i32 [[X]], 4
+; CHECK-NEXT: call void @llvm.assume(i1 [[ICMP]])
+; CHECK-NEXT: [[Y:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 [[X]], i32 3, i1 false)
+; CHECK-NEXT: ret i32 [[Y]]
+;
+ %icmp = icmp ule i32 %x, 3
+ call void @llvm.assume(i1 %icmp)
+ %y = call i32 @llvm.experimental.get.vector.length(i32 %x, i32 3, i1 false)
+ ret i32 %y
+}
+
+define i32 @cnt_known_lt_runtime_trunc(i64 %x) {
+; CHECK-LABEL: define i32 @cnt_known_lt_runtime_trunc(
+; CHECK-SAME: i64 [[X:%.*]]) {
+; CHECK-NEXT: [[ICMP:%.*]] = icmp ult i64 [[X]], 4
+; CHECK-NEXT: call void @llvm.assume(i1 [[ICMP]])
+; CHECK-NEXT: [[Y:%.*]] = call i32 @llvm.experimental.get.vector.length.i64(i64 [[X]], i32 3, i1 false)
+; CHECK-NEXT: ret i32 [[Y]]
+;
+ %icmp = icmp ule i64 %x, 3
+ call void @llvm.assume(i1 %icmp)
+ %y = call i32 @llvm.experimental.get.vector.length(i64 %x, i32 3, i1 false)
+ ret i32 %y
+}
+
+; FIXME: We should be able to deduce the constant range from AssumptionCache
+; rather than relying on KnownBits, which in this case only knows x <= 3.
+define i32 @cnt_known_lt_runtime_assumption(i32 %x) {
+; CHECK-LABEL: define i32 @cnt_known_lt_runtime_assumption(
+; CHECK-SAME: i32 [[X:%.*]]) {
+; CHECK-NEXT: [[ICMP:%.*]] = icmp ult i32 [[X]], 3
+; CHECK-NEXT: call void @llvm.assume(i1 [[ICMP]])
+; CHECK-NEXT: [[Y:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 [[X]], i32 2, i1 false)
+; CHECK-NEXT: ret i32 [[Y]]
+;
+ %icmp = icmp ule i32 %x, 2
+ call void @llvm.assume(i1 %icmp)
+ %y = call i32 @llvm.experimental.get.vector.length(i32 %x, i32 2, i1 false)
+ ret i32 %y
+}
>From 4da2081d9743a9ae36ed3a93b04353930b1ad1ff Mon Sep 17 00:00:00 2001
From: Luke Lau <luke at igalia.com>
Date: Mon, 24 Nov 2025 16:16:01 +0800
Subject: [PATCH 2/4] [InstCombine] Fold @llvm.experimental.get.vector.length
when cnt <= max_lanes
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, but always truncates the result to i32).
Note that there was something similar done in VPlan in #167647 for when the loop vectorizer knows the trip count.
---
.../Transforms/InstCombine/InstCombineCalls.cpp | 16 ++++++++++++++++
.../Transforms/InstCombine/get_vector_length.ll | 11 ++++-------
2 files changed, 20 insertions(+), 7 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 8e4edefec42fd..247f615ed0b54 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -4005,6 +4005,22 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
}
break;
}
+ case Intrinsic::experimental_get_vector_length: {
+ // get.vector.length(Cnt, MaxLanes) --> Cnt when Cnt <= MaxLanes
+ ConstantRange Cnt = computeConstantRangeIncludingKnownBits(
+ II->getArgOperand(0), false, SQ.getWithInstruction(II));
+ ConstantRange MaxLanes = cast<ConstantInt>(II->getArgOperand(1))
+ ->getValue()
+ .zext(Cnt.getBitWidth());
+ if (cast<ConstantInt>(II->getArgOperand(2))->getZExtValue())
+ MaxLanes = MaxLanes.multiply(
+ getVScaleRange(II->getFunction(), Cnt.getBitWidth()));
+
+ if (Cnt.icmp(CmpInst::ICMP_ULE, MaxLanes))
+ return replaceInstUsesWith(
+ *II, Builder.CreateTrunc(II->getArgOperand(0), II->getType()));
+ return nullptr;
+ }
default: {
// Handle target specific intrinsics
std::optional<Instruction *> V = targetInstCombineIntrinsic(*II);
diff --git a/llvm/test/Transforms/InstCombine/get_vector_length.ll b/llvm/test/Transforms/InstCombine/get_vector_length.ll
index 2925d6f556988..96a7f3058c43c 100644
--- a/llvm/test/Transforms/InstCombine/get_vector_length.ll
+++ b/llvm/test/Transforms/InstCombine/get_vector_length.ll
@@ -3,8 +3,7 @@
define i32 @cnt_known_lt() {
; CHECK-LABEL: define i32 @cnt_known_lt() {
-; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 1, i32 2, i1 false)
-; CHECK-NEXT: ret i32 [[X]]
+; CHECK-NEXT: ret i32 1
;
%x = call i32 @llvm.experimental.get.vector.length(i32 1, i32 2, i1 false)
ret i32 %x
@@ -22,8 +21,7 @@ define i32 @cnt_not_known_lt() {
define i32 @cnt_known_lt_scalable() vscale_range(2, 4) {
; CHECK-LABEL: define i32 @cnt_known_lt_scalable(
; CHECK-SAME: ) #[[ATTR0:[0-9]+]] {
-; CHECK-NEXT: [[X:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 2, i32 1, i1 true)
-; CHECK-NEXT: ret i32 [[X]]
+; CHECK-NEXT: ret i32 2
;
%x = call i32 @llvm.experimental.get.vector.length(i32 2, i32 1, i1 true)
ret i32 %x
@@ -43,8 +41,7 @@ define i32 @cnt_known_lt_runtime(i32 %x) {
; CHECK-SAME: i32 [[X:%.*]]) {
; CHECK-NEXT: [[ICMP:%.*]] = icmp ult i32 [[X]], 4
; CHECK-NEXT: call void @llvm.assume(i1 [[ICMP]])
-; CHECK-NEXT: [[Y:%.*]] = call i32 @llvm.experimental.get.vector.length.i32(i32 [[X]], i32 3, i1 false)
-; CHECK-NEXT: ret i32 [[Y]]
+; CHECK-NEXT: ret i32 [[X]]
;
%icmp = icmp ule i32 %x, 3
call void @llvm.assume(i1 %icmp)
@@ -57,7 +54,7 @@ define i32 @cnt_known_lt_runtime_trunc(i64 %x) {
; CHECK-SAME: i64 [[X:%.*]]) {
; CHECK-NEXT: [[ICMP:%.*]] = icmp ult i64 [[X]], 4
; CHECK-NEXT: call void @llvm.assume(i1 [[ICMP]])
-; CHECK-NEXT: [[Y:%.*]] = call i32 @llvm.experimental.get.vector.length.i64(i64 [[X]], i32 3, i1 false)
+; CHECK-NEXT: [[Y:%.*]] = trunc nuw nsw i64 [[X]] to i32
; CHECK-NEXT: ret i32 [[Y]]
;
%icmp = icmp ule i64 %x, 3
>From 2f71e8a38242da1740c8820d659e0a4ea95e5f2f Mon Sep 17 00:00:00 2001
From: Luke Lau <luke at igalia.com>
Date: Tue, 25 Nov 2025 18:21:51 +0800
Subject: [PATCH 3/4] Use createZExtOrTrunc, use isOne()
---
llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp | 4 ++--
1 file changed, 2 insertions(+), 2 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index 247f615ed0b54..a576dc42f43b9 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -4012,13 +4012,13 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
ConstantRange MaxLanes = cast<ConstantInt>(II->getArgOperand(1))
->getValue()
.zext(Cnt.getBitWidth());
- if (cast<ConstantInt>(II->getArgOperand(2))->getZExtValue())
+ if (cast<ConstantInt>(II->getArgOperand(2))->isOne())
MaxLanes = MaxLanes.multiply(
getVScaleRange(II->getFunction(), Cnt.getBitWidth()));
if (Cnt.icmp(CmpInst::ICMP_ULE, MaxLanes))
return replaceInstUsesWith(
- *II, Builder.CreateTrunc(II->getArgOperand(0), II->getType()));
+ *II, Builder.CreateZExtOrTrunc(II->getArgOperand(0), II->getType()));
return nullptr;
}
default: {
>From db24622342e6abf7e15672aca9fe7bb8658c81c5 Mon Sep 17 00:00:00 2001
From: Luke Lau <luke at igalia.com>
Date: Wed, 26 Nov 2025 16:15:12 +0800
Subject: [PATCH 4/4] Perform in larger of two bitwidths
---
llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp | 11 ++++++++---
llvm/test/Transforms/InstCombine/get_vector_length.ll | 9 +++++++++
2 files changed, 17 insertions(+), 3 deletions(-)
diff --git a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
index a576dc42f43b9..ad6407fba81da 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineCalls.cpp
@@ -4007,11 +4007,16 @@ Instruction *InstCombinerImpl::visitCallInst(CallInst &CI) {
}
case Intrinsic::experimental_get_vector_length: {
// get.vector.length(Cnt, MaxLanes) --> Cnt when Cnt <= MaxLanes
- ConstantRange Cnt = computeConstantRangeIncludingKnownBits(
- II->getArgOperand(0), false, SQ.getWithInstruction(II));
+ unsigned BitWidth =
+ std::max(II->getArgOperand(0)->getType()->getScalarSizeInBits(),
+ II->getType()->getScalarSizeInBits());
+ ConstantRange Cnt =
+ computeConstantRangeIncludingKnownBits(II->getArgOperand(0), false,
+ SQ.getWithInstruction(II))
+ .zextOrTrunc(BitWidth);
ConstantRange MaxLanes = cast<ConstantInt>(II->getArgOperand(1))
->getValue()
- .zext(Cnt.getBitWidth());
+ .zextOrTrunc(Cnt.getBitWidth());
if (cast<ConstantInt>(II->getArgOperand(2))->isOne())
MaxLanes = MaxLanes.multiply(
getVScaleRange(II->getFunction(), Cnt.getBitWidth()));
diff --git a/llvm/test/Transforms/InstCombine/get_vector_length.ll b/llvm/test/Transforms/InstCombine/get_vector_length.ll
index 96a7f3058c43c..122beeae866f3 100644
--- a/llvm/test/Transforms/InstCombine/get_vector_length.ll
+++ b/llvm/test/Transforms/InstCombine/get_vector_length.ll
@@ -78,3 +78,12 @@ define i32 @cnt_known_lt_runtime_assumption(i32 %x) {
%y = call i32 @llvm.experimental.get.vector.length(i32 %x, i32 2, i1 false)
ret i32 %y
}
+
+
+define i32 @cnt_known_lt_i16() {
+; CHECK-LABEL: define i32 @cnt_known_lt_i16() {
+; CHECK-NEXT: ret i32 1
+;
+ %x = call i32 @llvm.experimental.get.vector.length(i16 1, i32 2, i1 false)
+ ret i32 %x
+}
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