[instcombine] Scalarize operands of vector geps if possible #145402

preames · 2025-06-23T20:28:39Z

If we have a gep with vector indices which were splats (either constants or shuffles), prefer the scalar form of the index. If all operands are scalarizable, then prefer a scalar gep with splat following.

This does loose some information about undef/poison lanes, but I'm not sure that's significant versus the number of downstream transformations which get confused by having to manual scalarize operands.

If we have a gep with vector indices which were splats (either constants or shuffles), prefer the scalar form of the index. If all operands are scalarizable, then prefer a scalar gep with splat following. This does loose some information about undef/poison lanes, but I'm not sure that's significant versus the number of downstream transformations which get confused by having to manual scalarize operands.

llvmbot · 2025-06-23T20:29:18Z

@llvm/pr-subscribers-llvm-transforms

Author: Philip Reames (preames)

Changes

If we have a gep with vector indices which were splats (either constants or shuffles), prefer the scalar form of the index. If all operands are scalarizable, then prefer a scalar gep with splat following.

This does loose some information about undef/poison lanes, but I'm not sure that's significant versus the number of downstream transformations which get confused by having to manual scalarize operands.

Full diff: https://github.com/llvm/llvm-project/pull/145402.diff

7 Files Affected:

(modified) llvm/lib/Transforms/InstCombine/InstructionCombining.cpp (+25)
(modified) llvm/test/Transforms/InstCombine/fold-phi-arg-gep-to-phi-negative.ll (+3-3)
(modified) llvm/test/Transforms/InstCombine/getelementptr.ll (+5-5)
(modified) llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll (+9-15)
(modified) llvm/test/Transforms/InstCombine/vec_demanded_elts.ll (+9-15)
(modified) llvm/test/Transforms/InstCombine/vector_gep1-inseltpoison.ll (+1-1)
(modified) llvm/test/Transforms/InstCombine/vector_gep1.ll (+1-1)

diff --git a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
index ce42029261359..854dd358b1a8c 100644
--- a/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstructionCombining.cpp
@@ -3058,6 +3058,31 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
     return replaceInstUsesWith(GEP, NewGEP);
   }
 
+  // Scalarize vector operands; prefer splat-of-gep.as canonical form.
+  if (GEPType->isVectorTy() && llvm::any_of(GEP.operands(), [](Value *Op) {
+        return Op->getType()->isVectorTy() && getSplatValue(Op);
+      })) {
+    SmallVector<Value *> NewOps;
+    for (auto &Op : GEP.operands()) {
+      if (Op->getType()->isVectorTy())
+        if (Value *Scalar = getSplatValue(Op)) {
+          NewOps.push_back(Scalar);
+          continue;
+        }
+      NewOps.push_back(Op);
+    }
+
+    Value *Res = Builder.CreateGEP(GEP.getSourceElementType(), NewOps[0],
+                                   ArrayRef(NewOps).drop_front(), GEP.getName(),
+                                   GEP.getNoWrapFlags());
+    if (llvm::all_of(NewOps,
+                     [](Value *Op) { return !Op->getType()->isVectorTy(); })) {
+      ElementCount EC = cast<VectorType>(GEPType)->getElementCount();
+      Res = Builder.CreateVectorSplat(EC, Res);
+    }
+    return replaceInstUsesWith(GEP, Res);
+  }
+
   // Check to see if the inputs to the PHI node are getelementptr instructions.
   if (auto *PN = dyn_cast<PHINode>(PtrOp)) {
     if (Value *NewPtrOp = foldGEPOfPhi(GEP, PN, Builder))
diff --git a/llvm/test/Transforms/InstCombine/fold-phi-arg-gep-to-phi-negative.ll b/llvm/test/Transforms/InstCombine/fold-phi-arg-gep-to-phi-negative.ll
index 0bbb1035b1093..c5f9dc1861704 100644
--- a/llvm/test/Transforms/InstCombine/fold-phi-arg-gep-to-phi-negative.ll
+++ b/llvm/test/Transforms/InstCombine/fold-phi-arg-gep-to-phi-negative.ll
@@ -11,13 +11,13 @@ define <16 x ptr> @test(i1 %tobool) {
 ; CHECK-NEXT:    [[LANE_15:%.*]] = insertelement <16 x ptr> poison, ptr [[LANE_0]], i64 0
 ; CHECK-NEXT:    br i1 [[TOBOOL]], label %[[F1:.*]], label %[[F0:.*]]
 ; CHECK:       [[F0]]:
-; CHECK-NEXT:    [[MM_VECTORGEP:%.*]] = getelementptr inbounds [[FOO]], <16 x ptr> [[LANE_15]], <16 x i64> zeroinitializer, <16 x i32> splat (i32 1)
+; CHECK-NEXT:    [[MM_VECTORGEP1:%.*]] = getelementptr inbounds i8, <16 x ptr> [[LANE_15]], i64 2
 ; CHECK-NEXT:    br label %[[MERGE:.*]]
 ; CHECK:       [[F1]]:
-; CHECK-NEXT:    [[MM_VECTORGEP2:%.*]] = getelementptr inbounds [[FOO]], <16 x ptr> [[LANE_15]], <16 x i64> zeroinitializer, <16 x i32> splat (i32 2)
+; CHECK-NEXT:    [[MM_VECTORGEP22:%.*]] = getelementptr inbounds i8, <16 x ptr> [[LANE_15]], i64 4
 ; CHECK-NEXT:    br label %[[MERGE]]
 ; CHECK:       [[MERGE]]:
-; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <16 x ptr> [ [[MM_VECTORGEP]], %[[F0]] ], [ [[MM_VECTORGEP2]], %[[F1]] ]
+; CHECK-NEXT:    [[VEC_PHI:%.*]] = phi <16 x ptr> [ [[MM_VECTORGEP1]], %[[F0]] ], [ [[MM_VECTORGEP22]], %[[F1]] ]
 ; CHECK-NEXT:    ret <16 x ptr> [[VEC_PHI]]
 ;
 entry:
diff --git a/llvm/test/Transforms/InstCombine/getelementptr.ll b/llvm/test/Transforms/InstCombine/getelementptr.ll
index 7568c6edc429c..b7d263f5eb320 100644
--- a/llvm/test/Transforms/InstCombine/getelementptr.ll
+++ b/llvm/test/Transforms/InstCombine/getelementptr.ll
@@ -247,11 +247,11 @@ define <2 x i1> @test13_vector2(i64 %X, <2 x ptr> %P) nounwind {
 
 define <2 x i1> @test13_fixed_fixed(i64 %X, ptr %P, <2 x i64> %y) nounwind {
 ; CHECK-LABEL: @test13_fixed_fixed(
-; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <2 x i64> poison, i64 [[X:%.*]], i64 0
-; CHECK-NEXT:    [[TMP1:%.*]] = shl <2 x i64> [[DOTSPLATINSERT]], <i64 3, i64 0>
-; CHECK-NEXT:    [[A_IDX:%.*]] = shufflevector <2 x i64> [[TMP1]], <2 x i64> poison, <2 x i32> zeroinitializer
-; CHECK-NEXT:    [[B_IDX:%.*]] = shl nsw <2 x i64> [[Y:%.*]], splat (i64 4)
-; CHECK-NEXT:    [[C:%.*]] = icmp eq <2 x i64> [[A_IDX]], [[B_IDX]]
+; CHECK-NEXT:    [[A1:%.*]] = getelementptr inbounds <2 x i64>, ptr [[P:%.*]], i64 0, i64 [[X:%.*]]
+; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <2 x ptr> poison, ptr [[A1]], i64 0
+; CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <2 x ptr> [[DOTSPLATINSERT]], <2 x ptr> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[B:%.*]] = getelementptr inbounds <2 x i64>, ptr [[P]], <2 x i64> [[Y:%.*]]
+; CHECK-NEXT:    [[C:%.*]] = icmp eq <2 x ptr> [[DOTSPLAT]], [[B]]
 ; CHECK-NEXT:    ret <2 x i1> [[C]]
 ;
   %A = getelementptr inbounds <2 x i64>, ptr %P, <2 x i64> zeroinitializer, i64 %X
diff --git a/llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll b/llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll
index a240dfe7d271a..cbfdebc6cb412 100644
--- a/llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll
+++ b/llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll
@@ -525,9 +525,7 @@ define ptr @gep_splat_base_w_s_idx(ptr %base) {
 
 define ptr @gep_splat_base_w_cv_idx(ptr %base) {
 ; CHECK-LABEL: @gep_splat_base_w_cv_idx(
-; CHECK-NEXT:    [[BASEVEC2:%.*]] = insertelement <2 x ptr> poison, ptr [[BASE:%.*]], i64 1
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i32, <2 x ptr> [[BASEVEC2]], <2 x i64> <i64 poison, i64 1>
-; CHECK-NEXT:    [[EE:%.*]] = extractelement <2 x ptr> [[GEP]], i64 1
+; CHECK-NEXT:    [[EE:%.*]] = getelementptr i8, ptr [[BASE:%.*]], i64 4
 ; CHECK-NEXT:    ret ptr [[EE]]
 ;
   %basevec1 = insertelement <2 x ptr> poison, ptr %base, i32 0
@@ -539,9 +537,8 @@ define ptr @gep_splat_base_w_cv_idx(ptr %base) {
 
 define ptr @gep_splat_base_w_vidx(ptr %base, <2 x i64> %idxvec) {
 ; CHECK-LABEL: @gep_splat_base_w_vidx(
-; CHECK-NEXT:    [[BASEVEC2:%.*]] = insertelement <2 x ptr> poison, ptr [[BASE:%.*]], i64 1
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i32, <2 x ptr> [[BASEVEC2]], <2 x i64> [[IDXVEC:%.*]]
-; CHECK-NEXT:    [[EE:%.*]] = extractelement <2 x ptr> [[GEP]], i64 1
+; CHECK-NEXT:    [[TMP1:%.*]] = extractelement <2 x i64> [[IDXVEC:%.*]], i64 1
+; CHECK-NEXT:    [[EE:%.*]] = getelementptr i32, ptr [[BASE:%.*]], i64 [[TMP1]]
 ; CHECK-NEXT:    ret ptr [[EE]]
 ;
   %basevec1 = insertelement <2 x ptr> poison, ptr %base, i32 0
@@ -597,10 +594,7 @@ define ptr @gep_sbase_w_splat_idx(ptr %base, i64 %idx) {
 }
 define ptr @gep_splat_both(ptr %base, i64 %idx) {
 ; CHECK-LABEL: @gep_splat_both(
-; CHECK-NEXT:    [[BASEVEC2:%.*]] = insertelement <2 x ptr> poison, ptr [[BASE:%.*]], i64 1
-; CHECK-NEXT:    [[IDXVEC2:%.*]] = insertelement <2 x i64> poison, i64 [[IDX:%.*]], i64 1
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i32, <2 x ptr> [[BASEVEC2]], <2 x i64> [[IDXVEC2]]
-; CHECK-NEXT:    [[EE:%.*]] = extractelement <2 x ptr> [[GEP]], i64 1
+; CHECK-NEXT:    [[EE:%.*]] = getelementptr i32, ptr [[BASE:%.*]], i64 [[IDX:%.*]]
 ; CHECK-NEXT:    ret ptr [[EE]]
 ;
   %basevec1 = insertelement <2 x ptr> poison, ptr %base, i32 0
@@ -641,9 +635,9 @@ define ptr @gep_demanded_lane_undef(ptr %base, i64 %idx) {
 ;; indices.
 define ptr @PR41624(<2 x ptr> %a) {
 ; CHECK-LABEL: @PR41624(
-; CHECK-NEXT:    [[W:%.*]] = getelementptr { i32, i32 }, <2 x ptr> [[A:%.*]], <2 x i64> splat (i64 5), <2 x i32> zeroinitializer
-; CHECK-NEXT:    [[R:%.*]] = extractelement <2 x ptr> [[W]], i64 0
-; CHECK-NEXT:    ret ptr [[R]]
+; CHECK-NEXT:    [[R:%.*]] = extractelement <2 x ptr> [[W:%.*]], i64 0
+; CHECK-NEXT:    [[R1:%.*]] = getelementptr i8, ptr [[R]], i64 40
+; CHECK-NEXT:    ret ptr [[R1]]
 ;
   %w = getelementptr { i32, i32 }, <2 x ptr> %a, <2 x i64> <i64 5, i64 5>, <2 x i32> zeroinitializer
   %r = extractelement <2 x ptr> %w, i32 0
@@ -657,8 +651,8 @@ define ptr @PR41624(<2 x ptr> %a) {
 define ptr @zero_sized_type_extract(<4 x i64> %arg, i64 %arg1) {
 ; CHECK-LABEL: @zero_sized_type_extract(
 ; CHECK-NEXT:  bb:
-; CHECK-NEXT:    [[T:%.*]] = getelementptr inbounds [0 x i32], <4 x ptr> <ptr @global, ptr poison, ptr poison, ptr poison>, <4 x i64> <i64 0, i64 poison, i64 poison, i64 poison>, <4 x i64> [[ARG:%.*]]
-; CHECK-NEXT:    [[T2:%.*]] = extractelement <4 x ptr> [[T]], i64 0
+; CHECK-NEXT:    [[TMP0:%.*]] = extractelement <4 x i64> [[ARG:%.*]], i64 0
+; CHECK-NEXT:    [[T2:%.*]] = getelementptr inbounds [0 x i32], ptr @global, i64 0, i64 [[TMP0]]
 ; CHECK-NEXT:    ret ptr [[T2]]
 ;
 bb:
diff --git a/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll b/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll
index ee7ef9955e643..f0ef33dcc6b4e 100644
--- a/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll
+++ b/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll
@@ -528,9 +528,7 @@ define ptr @gep_splat_base_w_s_idx(ptr %base) {
 
 define ptr @gep_splat_base_w_cv_idx(ptr %base) {
 ; CHECK-LABEL: @gep_splat_base_w_cv_idx(
-; CHECK-NEXT:    [[BASEVEC2:%.*]] = insertelement <2 x ptr> poison, ptr [[BASE:%.*]], i64 1
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i32, <2 x ptr> [[BASEVEC2]], <2 x i64> <i64 poison, i64 1>
-; CHECK-NEXT:    [[EE:%.*]] = extractelement <2 x ptr> [[GEP]], i64 1
+; CHECK-NEXT:    [[EE:%.*]] = getelementptr i8, ptr [[BASE:%.*]], i64 4
 ; CHECK-NEXT:    ret ptr [[EE]]
 ;
   %basevec1 = insertelement <2 x ptr> undef, ptr %base, i32 0
@@ -542,9 +540,8 @@ define ptr @gep_splat_base_w_cv_idx(ptr %base) {
 
 define ptr @gep_splat_base_w_vidx(ptr %base, <2 x i64> %idxvec) {
 ; CHECK-LABEL: @gep_splat_base_w_vidx(
-; CHECK-NEXT:    [[BASEVEC2:%.*]] = insertelement <2 x ptr> poison, ptr [[BASE:%.*]], i64 1
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i32, <2 x ptr> [[BASEVEC2]], <2 x i64> [[IDXVEC:%.*]]
-; CHECK-NEXT:    [[EE:%.*]] = extractelement <2 x ptr> [[GEP]], i64 1
+; CHECK-NEXT:    [[TMP1:%.*]] = extractelement <2 x i64> [[IDXVEC:%.*]], i64 1
+; CHECK-NEXT:    [[EE:%.*]] = getelementptr i32, ptr [[BASE:%.*]], i64 [[TMP1]]
 ; CHECK-NEXT:    ret ptr [[EE]]
 ;
   %basevec1 = insertelement <2 x ptr> undef, ptr %base, i32 0
@@ -600,10 +597,7 @@ define ptr @gep_sbase_w_splat_idx(ptr %base, i64 %idx) {
 }
 define ptr @gep_splat_both(ptr %base, i64 %idx) {
 ; CHECK-LABEL: @gep_splat_both(
-; CHECK-NEXT:    [[BASEVEC2:%.*]] = insertelement <2 x ptr> poison, ptr [[BASE:%.*]], i64 1
-; CHECK-NEXT:    [[IDXVEC2:%.*]] = insertelement <2 x i64> poison, i64 [[IDX:%.*]], i64 1
-; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i32, <2 x ptr> [[BASEVEC2]], <2 x i64> [[IDXVEC2]]
-; CHECK-NEXT:    [[EE:%.*]] = extractelement <2 x ptr> [[GEP]], i64 1
+; CHECK-NEXT:    [[EE:%.*]] = getelementptr i32, ptr [[BASE:%.*]], i64 [[IDX:%.*]]
 ; CHECK-NEXT:    ret ptr [[EE]]
 ;
   %basevec1 = insertelement <2 x ptr> undef, ptr %base, i32 0
@@ -644,9 +638,9 @@ define ptr @gep_demanded_lane_undef(ptr %base, i64 %idx) {
 ;; indices.
 define ptr @PR41624(<2 x ptr> %a) {
 ; CHECK-LABEL: @PR41624(
-; CHECK-NEXT:    [[W:%.*]] = getelementptr { i32, i32 }, <2 x ptr> [[A:%.*]], <2 x i64> splat (i64 5), <2 x i32> zeroinitializer
-; CHECK-NEXT:    [[R:%.*]] = extractelement <2 x ptr> [[W]], i64 0
-; CHECK-NEXT:    ret ptr [[R]]
+; CHECK-NEXT:    [[R:%.*]] = extractelement <2 x ptr> [[W:%.*]], i64 0
+; CHECK-NEXT:    [[R1:%.*]] = getelementptr i8, ptr [[R]], i64 40
+; CHECK-NEXT:    ret ptr [[R1]]
 ;
   %w = getelementptr { i32, i32 }, <2 x ptr> %a, <2 x i64> <i64 5, i64 5>, <2 x i32> zeroinitializer
   %r = extractelement <2 x ptr> %w, i32 0
@@ -660,8 +654,8 @@ define ptr @PR41624(<2 x ptr> %a) {
 define ptr @zero_sized_type_extract(<4 x i64> %arg, i64 %arg1) {
 ; CHECK-LABEL: @zero_sized_type_extract(
 ; CHECK-NEXT:  bb:
-; CHECK-NEXT:    [[T:%.*]] = getelementptr inbounds [0 x i32], <4 x ptr> <ptr @global, ptr poison, ptr poison, ptr poison>, <4 x i64> <i64 0, i64 poison, i64 poison, i64 poison>, <4 x i64> [[ARG:%.*]]
-; CHECK-NEXT:    [[T2:%.*]] = extractelement <4 x ptr> [[T]], i64 0
+; CHECK-NEXT:    [[TMP0:%.*]] = extractelement <4 x i64> [[ARG:%.*]], i64 0
+; CHECK-NEXT:    [[T2:%.*]] = getelementptr inbounds [0 x i32], ptr @global, i64 0, i64 [[TMP0]]
 ; CHECK-NEXT:    ret ptr [[T2]]
 ;
 bb:
diff --git a/llvm/test/Transforms/InstCombine/vector_gep1-inseltpoison.ll b/llvm/test/Transforms/InstCombine/vector_gep1-inseltpoison.ll
index 994ecc2246bb6..b882d7dfff91e 100644
--- a/llvm/test/Transforms/InstCombine/vector_gep1-inseltpoison.ll
+++ b/llvm/test/Transforms/InstCombine/vector_gep1-inseltpoison.ll
@@ -55,7 +55,7 @@ define <2 x i1> @test5(<2 x ptr> %a) {
 
 define <2 x ptr> @test7(<2 x ptr> %a) {
 ; CHECK-LABEL: @test7(
-; CHECK-NEXT:    [[W:%.*]] = getelementptr { i32, i32 }, <2 x ptr> [[A:%.*]], <2 x i64> <i64 5, i64 9>, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[W:%.*]] = getelementptr { i32, i32 }, <2 x ptr> [[A:%.*]], <2 x i64> <i64 5, i64 9>, i32 0
 ; CHECK-NEXT:    ret <2 x ptr> [[W]]
 ;
   %w = getelementptr {i32, i32}, <2 x ptr> %a, <2 x i32> <i32 5, i32 9>, <2 x i32> zeroinitializer
diff --git a/llvm/test/Transforms/InstCombine/vector_gep1.ll b/llvm/test/Transforms/InstCombine/vector_gep1.ll
index ab99ee71fc8db..92de12213bee1 100644
--- a/llvm/test/Transforms/InstCombine/vector_gep1.ll
+++ b/llvm/test/Transforms/InstCombine/vector_gep1.ll
@@ -55,7 +55,7 @@ define <2 x i1> @test5(<2 x ptr> %a) {
 
 define <2 x ptr> @test7(<2 x ptr> %a) {
 ; CHECK-LABEL: @test7(
-; CHECK-NEXT:    [[W:%.*]] = getelementptr { i32, i32 }, <2 x ptr> [[A:%.*]], <2 x i64> <i64 5, i64 9>, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[W:%.*]] = getelementptr { i32, i32 }, <2 x ptr> [[A:%.*]], <2 x i64> <i64 5, i64 9>, i32 0
 ; CHECK-NEXT:    ret <2 x ptr> [[W]]
 ;
   %w = getelementptr {i32, i32}, <2 x ptr> %a, <2 x i32> <i32 5, i32 9>, <2 x i32> zeroinitializer

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

nikic

LGTM

nikic · 2025-06-24T15:19:00Z

llvm/lib/Transforms/InstCombine/InstructionCombining.cpp

@@ -2983,6 +2983,7 @@ Instruction *InstCombinerImpl::visitGetElementPtrInst(GetElementPtrInst &GEP) {
  // For vector geps, use the generic demanded vector support.
  // Skip if GEP return type is scalable. The number of elements is unknown at
  // compile-time.
+  // TODO: exploit undef elements to decrease demanded bits


I don't understand what you mean by this TODO.

I was simply moving the comment. I think it refers to using undef elements of one index to reason that the same elements of another index are undemanded since the result for that lane must be undefined. (Hm, having written that, undef seems slightly questionable, but it should apply for poison)

I decided to just remove the comment for the moment. A variant of it might belong in the demanded elts logic itself, but even if so, this is the wrong spot for it.

nikic · 2025-06-24T15:22:26Z

llvm/test/Transforms/InstCombine/getelementptr.ll

+; CHECK-NEXT:    [[DOTSPLATINSERT:%.*]] = insertelement <2 x ptr> poison, ptr [[A1]], i64 0
+; CHECK-NEXT:    [[DOTSPLAT:%.*]] = shufflevector <2 x ptr> [[DOTSPLATINSERT]], <2 x ptr> poison, <2 x i32> zeroinitializer
+; CHECK-NEXT:    [[B:%.*]] = getelementptr inbounds <2 x i64>, ptr [[P]], <2 x i64> [[Y:%.*]]
+; CHECK-NEXT:    [[C:%.*]] = icmp eq <2 x ptr> [[DOTSPLAT]], [[B]]


Regression, as we don't look through splats in this transform. But I doubt this matters in practice.

preames · 2025-06-24T19:21:56Z

Hm, I'm now wondering if this patch is a good idea. I went to do an obvious follow up - matching "insetelt poison, V, 0" in getSplatValue - and started seeing a bunch of regressions in tests. Digging into it, it seems to come from two sources, one fixable, one hard. The first is that we have a bunch of combines which don't look through splats, even when they easily could. The second is the demanded lane piece I'd mentioned in the original review.

The demanded bits one is quite tricky to resolve, with the scalarization we don't have anywhere to record which lanes are poison. There's also a bunch of concerning comments in the inference code about a partially completed undef to poison transition which seemed like a ball of twine to deal with.

I'm tempted to leave this in, and see if we get any reports of actual regressions on non-contrived cases. Anyone disagree?

) If we have a gep with vector indices which were splats (either constants or shuffles), prefer the scalar form of the index. If all operands are scalarizable, then prefer a scalar gep with splat following. This does loose some information about undef/poison lanes, but I'm not sure that's significant versus the number of downstream transformations which get confused by having to manual scalarize operands.

preames requested a review from dtcxzyw June 23, 2025 20:28

preames requested a review from nikic as a code owner June 23, 2025 20:28

llvmbot added llvm:instcombine Covers the InstCombine, InstSimplify and AggressiveInstCombine passes llvm:transforms labels Jun 23, 2025

dtcxzyw reviewed Jun 24, 2025

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Fuzz PR145402 dtcxzyw/llvm-mutation-based-fuzz-service#66

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Fuzz PR145402 dtcxzyw/llvm-fuzz-service#89

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nikic approved these changes Jun 24, 2025

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preames merged commit a848916 into llvm:main Jun 24, 2025
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preames deleted the pr-instcombine-scalarize-gep-operands branch June 24, 2025 16:48

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