[InstSimplify] Add basic simplifications for vp.reverse

[preames]

Directly modeled after what we do for vector.reverse, but with restrictions on EVL and mask added.

[llvmbot]

@llvm/pr-subscribers-llvm-transforms

@llvm/pr-subscribers-llvm-analysis

Author: Philip Reames (preames)

Changes

Directly modeled after what we do for vector.reverse, but with restrictions on EVL and mask added.

---

Full diff: https://github.com/llvm/llvm-project/pull/144112.diff

2 Files Affected:

• (modified) llvm/lib/Analysis/InstructionSimplify.cpp (+17)
• (modified) llvm/test/Transforms/InstSimplify/vp-reverse.ll (+6-13)

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index e397a228afee0..d1ac8d9fbdfd1 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/llvm/lib/Analysis/InstructionSimplify.cpp
@@ -6969,6 +6969,23 @@ static Value *simplifyIntrinsic(CallBase *Call, Value *Callee,
     }
     return nullptr;
   }
+  case Intrinsic::experimental_vp_reverse: {
+    Value *Vec = Call->getArgOperand(0);
+    Value *Mask = Call->getArgOperand(1);
+    Value *EVL = Call->getArgOperand(2);
+
+    Value *X;
+    // vp.reverse(vp.reverse(X)) == X (with all ones mask and matching EVL)
+    if (match(Mask, m_AllOnes()) &&
+        match(Vec, m_Intrinsic<Intrinsic::experimental_vp_reverse>(
+                       m_Value(X), m_AllOnes(), m_Specific(EVL))))
+      return X;
+
+    // vp.reverse(splat(X)) -> splat(X) (regardless of mask and EVL)
+    if (isSplatValue(Vec))
+      return Vec;
+    return nullptr;
+  }
   default:
     return nullptr;
   }
diff --git a/llvm/test/Transforms/InstSimplify/vp-reverse.ll b/llvm/test/Transforms/InstSimplify/vp-reverse.ll
index 3c3bb871dc610..f19a2ac8ca9e1 100644
--- a/llvm/test/Transforms/InstSimplify/vp-reverse.ll
+++ b/llvm/test/Transforms/InstSimplify/vp-reverse.ll
@@ -3,9 +3,7 @@
 
 define <vscale x 4 x i32> @rev_of_rev(<vscale x 4 x i32> %a, i32 %evl) {
 ; CHECK-LABEL: @rev_of_rev(
-; CHECK-NEXT:    [[A_REV:%.*]] = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse.nxv4i32(<vscale x 4 x i32> [[A:%.*]], <vscale x 4 x i1> splat (i1 true), i32 [[EVL:%.*]])
-; CHECK-NEXT:    [[RES:%.*]] = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse.nxv4i32(<vscale x 4 x i32> [[A_REV]], <vscale x 4 x i1> splat (i1 true), i32 [[EVL]])
-; CHECK-NEXT:    ret <vscale x 4 x i32> [[RES]]
+; CHECK-NEXT:    ret <vscale x 4 x i32> [[A:%.*]]
 ;
   %a.rev = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse(<vscale x 4 x i32> %a, <vscale x 4 x i1> splat (i1 true), i32 %evl)
   %res = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse(<vscale x 4 x i32> %a.rev, <vscale x 4 x i1> splat (i1 true), i32 %evl)
@@ -25,8 +23,7 @@ define <vscale x 4 x i32> @rev_of_rev_diffevl(<vscale x 4 x i32> %a, i32 %evl) {
 
 define <vscale x 4 x i32> @rev_of_poison(i32 %evl) {
 ; CHECK-LABEL: @rev_of_poison(
-; CHECK-NEXT:    [[REV:%.*]] = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse.nxv4i32(<vscale x 4 x i32> poison, <vscale x 4 x i1> splat (i1 true), i32 [[EVL:%.*]])
-; CHECK-NEXT:    ret <vscale x 4 x i32> [[REV]]
+; CHECK-NEXT:    ret <vscale x 4 x i32> poison
 ;
   %rev = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse(<vscale x 4 x i32> poison, <vscale x 4 x i1> splat (i1 true), i32 %evl)
   ret <vscale x 4 x i32> %rev
@@ -34,8 +31,7 @@ define <vscale x 4 x i32> @rev_of_poison(i32 %evl) {
 
 define <vscale x 4 x i32> @rev_of_undef(i32 %evl) {
 ; CHECK-LABEL: @rev_of_undef(
-; CHECK-NEXT:    [[REV:%.*]] = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse.nxv4i32(<vscale x 4 x i32> undef, <vscale x 4 x i1> splat (i1 true), i32 [[EVL:%.*]])
-; CHECK-NEXT:    ret <vscale x 4 x i32> [[REV]]
+; CHECK-NEXT:    ret <vscale x 4 x i32> undef
 ;
   %rev = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse(<vscale x 4 x i32> undef, <vscale x 4 x i1> splat (i1 true), i32 %evl)
   ret <vscale x 4 x i32> %rev
@@ -43,8 +39,7 @@ define <vscale x 4 x i32> @rev_of_undef(i32 %evl) {
 
 define <vscale x 4 x i32> @rev_of_zero(i32 %evl) {
 ; CHECK-LABEL: @rev_of_zero(
-; CHECK-NEXT:    [[REV:%.*]] = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse.nxv4i32(<vscale x 4 x i32> zeroinitializer, <vscale x 4 x i1> splat (i1 true), i32 [[EVL:%.*]])
-; CHECK-NEXT:    ret <vscale x 4 x i32> [[REV]]
+; CHECK-NEXT:    ret <vscale x 4 x i32> zeroinitializer
 ;
   %rev = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse(<vscale x 4 x i32> zeroinitializer, <vscale x 4 x i1> splat (i1 true), i32 %evl)
   ret <vscale x 4 x i32> %rev
@@ -54,8 +49,7 @@ define <vscale x 4 x i32> @rev_of_splat(i32 %a, i32 %evl) {
 ; CHECK-LABEL: @rev_of_splat(
 ; CHECK-NEXT:    [[A_INS:%.*]] = insertelement <vscale x 4 x i32> poison, i32 [[A:%.*]], i32 0
 ; CHECK-NEXT:    [[A_VEC:%.*]] = shufflevector <vscale x 4 x i32> [[A_INS]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
-; CHECK-NEXT:    [[REV:%.*]] = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse.nxv4i32(<vscale x 4 x i32> [[A_VEC]], <vscale x 4 x i1> splat (i1 true), i32 [[EVL:%.*]])
-; CHECK-NEXT:    ret <vscale x 4 x i32> [[REV]]
+; CHECK-NEXT:    ret <vscale x 4 x i32> [[A_VEC]]
 ;
   %a.ins = insertelement <vscale x 4 x i32> poison, i32 %a, i32 0
   %a.vec = shufflevector <vscale x 4 x i32> %a.ins, <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
@@ -67,8 +61,7 @@ define <vscale x 4 x i32> @rev_of_splat2(i32 %a, <vscale x 4 x i1> %m, i32 %evl)
 ; CHECK-LABEL: @rev_of_splat2(
 ; CHECK-NEXT:    [[A_INS:%.*]] = insertelement <vscale x 4 x i32> poison, i32 [[A:%.*]], i32 0
 ; CHECK-NEXT:    [[A_VEC:%.*]] = shufflevector <vscale x 4 x i32> [[A_INS]], <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer
-; CHECK-NEXT:    [[REV:%.*]] = tail call <vscale x 4 x i32> @llvm.experimental.vp.reverse.nxv4i32(<vscale x 4 x i32> [[A_VEC]], <vscale x 4 x i1> [[M:%.*]], i32 [[EVL:%.*]])
-; CHECK-NEXT:    ret <vscale x 4 x i32> [[REV]]
+; CHECK-NEXT:    ret <vscale x 4 x i32> [[A_VEC]]
 ;
   %a.ins = insertelement <vscale x 4 x i32> poison, i32 %a, i32 0
   %a.vec = shufflevector <vscale x 4 x i32> %a.ins, <vscale x 4 x i32> poison, <vscale x 4 x i32> zeroinitializer

[github-actions]

⚠️ undef deprecator found issues in your code. ⚠️

You can test this locally with the following command:

git diff -U0 --pickaxe-regex -S '([^a-zA-Z0-9#_-]undef[^a-zA-Z0-9_-]|UndefValue::get)' 'HEAD~1' HEAD llvm/lib/Analysis/InstructionSimplify.cpp llvm/test/Transforms/InstSimplify/vp-reverse.ll

The following files introduce new uses of undef:

• llvm/test/Transforms/InstSimplify/vp-reverse.ll

Undef is now deprecated and should only be used in the rare cases where no replacement is possible. For example, a load of uninitialized memory yields undef. You should use poison values for placeholders instead.

In tests, avoid using undef and having tests that trigger undefined behavior. If you need an operand with some unimportant value, you can add a new argument to the function and use that instead.

For example, this is considered a bad practice:

define void @fn() {
  ...
  br i1 undef, ...
}

Please use the following instead:

define void @fn(i1 %cond) {
  ...
  br i1 %cond, ...
}

Please refer to the Undefined Behavior Manual for more information.

[dtcxzyw]

LG

[dtcxzyw]

We don't need undef tests. There are many PRs that drop undef tests...

[preames]

I want to push back a bit here. There's a big difference between removing undef from tests where the undef is the result of reduction, or doesn't reflect test intention. This case is specifically testing correct behavior of an undef fold. I would hope we are not removing such cases, that would seem actively undesirable.

[preames]

FYI, the CI was showing failures when I landed, but a) I built locally without issue, and b) the logs didn't actually show any test failures. I'm putting this down to a CI flak until proven otherwise.