[llvm] dd995ac - [InstCombine] remove incorrect gep(x, undef) -> undef optimization

[Nuno Lopes via llvm-commits]

Author: Nuno Lopes
Date: 2022-01-30T11:34:32Z
New Revision: dd995acedadfa27bca4e64f4cec4fa77c350adbe

URL: https://github.com/llvm/llvm-project/commit/dd995acedadfa27bca4e64f4cec4fa77c350adbe
DIFF: https://github.com/llvm/llvm-project/commit/dd995acedadfa27bca4e64f4cec4fa77c350adbe.diff

LOG: [InstCombine] remove incorrect gep(x, undef) -> undef optimization
gep(x, undef) carries the provenance of x, so we can't replace it with any
pointer like undef.
This leaves room for improvement for the poison case, but that's currently
not possible as the demanded bits API doesn't distinguish between undef &
poison bits.

Fixes #44790

Added: 
    

Modified: 
    llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
    llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll
    llvm/test/Transforms/InstCombine/vec_demanded_elts.ll

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
index 71a5ae24eead9..3f064cfda7122 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineSimplifyDemanded.cpp
@@ -1219,7 +1219,7 @@ Value *InstCombinerImpl::SimplifyDemandedVectorElts(Value *V,
       for (auto I = gep_type_begin(GEP), E = gep_type_end(GEP);
            I != E; I++)
         if (I.isStruct())
-          return true;;
+          return true;
       return false;
     };
     if (mayIndexStructType(cast<GetElementPtrInst>(*I)))
@@ -1228,10 +1228,11 @@ Value *InstCombinerImpl::SimplifyDemandedVectorElts(Value *V,
     // Conservatively track the demanded elements back through any vector
     // operands we may have.  We know there must be at least one, or we
     // wouldn't have a vector result to get here. Note that we intentionally
-    // merge the undef bits here since gepping with either an undef base or
-    // index results in undef.
+    // merge the undef bits here since gepping with either an poison base or
+    // index results in poison.
     for (unsigned i = 0; i < I->getNumOperands(); i++) {
-      if (match(I->getOperand(i), m_Undef())) {
+      if (i == 0 ? match(I->getOperand(i), m_Undef())
+                 : match(I->getOperand(i), m_Poison())) {
         // If the entire vector is undefined, just return this info.
         UndefElts = EltMask;
         return nullptr;
@@ -1239,7 +1240,11 @@ Value *InstCombinerImpl::SimplifyDemandedVectorElts(Value *V,
       if (I->getOperand(i)->getType()->isVectorTy()) {
         APInt UndefEltsOp(VWidth, 0);
         simplifyAndSetOp(I, i, DemandedElts, UndefEltsOp);
-        UndefElts |= UndefEltsOp;
+        // gep(x, undef) is not undef, so skip considering idx ops here
+        // Note that we could propagate poison, but we can't distinguish between
+        // undef & poison bits ATM
+        if (i == 0)
+          UndefElts |= UndefEltsOp;
       }
     }
 

diff  --git a/llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll b/llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll
index ad84f65f70a04..1c0cb17c62cff 100644
--- a/llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll
+++ b/llvm/test/Transforms/InstCombine/vec_demanded_elts-inseltpoison.ll
@@ -614,7 +614,10 @@ define i32* @gep_splat_both(i32* %base, i64 %idx) {
 
 define <2 x i32*> @gep_all_lanes_undef(i32* %base, i64 %idx) {;
 ; CHECK-LABEL: @gep_all_lanes_undef(
-; CHECK-NEXT:    ret <2 x i32*> undef
+; CHECK-NEXT:    [[BASEVEC:%.*]] = insertelement <2 x i32*> poison, i32* [[BASE:%.*]], i64 0
+; CHECK-NEXT:    [[IDXVEC:%.*]] = insertelement <2 x i64> poison, i64 [[IDX:%.*]], i64 1
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i32, <2 x i32*> [[BASEVEC]], <2 x i64> [[IDXVEC]]
+; CHECK-NEXT:    ret <2 x i32*> [[GEP]]
 ;
   %basevec = insertelement <2 x i32*> poison, i32* %base, i32 0
   %idxvec = insertelement <2 x i64> poison, i64 %idx, i32 1

diff  --git a/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll b/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll
index d64869f1041f0..17b3bdf979e4f 100644
--- a/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll
+++ b/llvm/test/Transforms/InstCombine/vec_demanded_elts.ll
@@ -614,7 +614,10 @@ define i32* @gep_splat_both(i32* %base, i64 %idx) {
 
 define <2 x i32*> @gep_all_lanes_undef(i32* %base, i64 %idx) {;
 ; CHECK-LABEL: @gep_all_lanes_undef(
-; CHECK-NEXT:    ret <2 x i32*> undef
+; CHECK-NEXT:    [[BASEVEC:%.*]] = insertelement <2 x i32*> undef, i32* [[BASE:%.*]], i64 0
+; CHECK-NEXT:    [[IDXVEC:%.*]] = insertelement <2 x i64> undef, i64 [[IDX:%.*]], i64 1
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr i32, <2 x i32*> [[BASEVEC]], <2 x i64> [[IDXVEC]]
+; CHECK-NEXT:    ret <2 x i32*> [[GEP]]
 ;
   %basevec = insertelement <2 x i32*> undef, i32* %base, i32 0
   %idxvec = insertelement <2 x i64> undef, i64 %idx, i32 1