[InstCombine] make icmp vector canonicalization safe for constant with undef elements

This is a fix for:
https://bugs.llvm.org/show_bug.cgi?id=43730
...and as shown there, we have existing test cases that show potential miscompiles.

We could just bail out for vector constants that contain any undef elements, or we can do as shown here:
allow the transform, but replace the undefs with a safe value.

For most of the tests shown, this results in a full splat constant (no undefs) which is probably a win
for further IR analysis because we conservatively don't match undefs in most cases. Codegen can probably
recover these kinds of undef lanes via demanded elements analysis if that's profitable.

Differential Revision: https://reviews.llvm.org/D69519

Author: rotateright

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

@@ -5167,6 +5167,7 @@ llvm::getFlippedStrictnessPredicateAndConstant(CmpInst::Predicate Pred,
     return WillIncrement ? !C->isMaxValue(IsSigned) : !C->isMinValue(IsSigned);
   };
 
+  Constant *SafeReplacementConstant = nullptr;
   if (auto *CI = dyn_cast<ConstantInt>(C)) {
     // Bail out if the constant can't be safely incremented/decremented.
     if (!ConstantIsOk(CI))
@@ -5186,12 +5187,23 @@ llvm::getFlippedStrictnessPredicateAndConstant(CmpInst::Predicate Pred,
       auto *CI = dyn_cast<ConstantInt>(Elt);
       if (!CI || !ConstantIsOk(CI))
         return llvm::None;
+
+      if (!SafeReplacementConstant)
+        SafeReplacementConstant = CI;
     }
   } else {
     // ConstantExpr?
     return llvm::None;
   }
 
+  // It may not be safe to change a compare predicate in the presence of
+  // undefined elements, so replace those elements with the first safe constant
+  // that we found.
+  if (C->containsUndefElement()) {
+    assert(SafeReplacementConstant && "Replacement constant not set");
+    C = Constant::replaceUndefsWith(C, SafeReplacementConstant);
+  }
+
   CmpInst::Predicate NewPred = CmpInst::getFlippedStrictnessPredicate(Pred);
 
   // Increment or decrement the constant.

llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-eq-to-icmp-ule.ll

@@ -82,7 +82,7 @@ define <2 x i1> @p2_vec_nonsplat_edgecase1(<2 x i8> %x) {
 
 define <3 x i1> @p3_vec_splat_undef(<3 x i8> %x) {
 ; CHECK-LABEL: @p3_vec_splat_undef(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult <3 x i8> [[X:%.*]], <i8 4, i8 undef, i8 4>
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult <3 x i8> [[X:%.*]], <i8 4, i8 4, i8 4>
 ; CHECK-NEXT:    ret <3 x i1> [[TMP1]]
 ;
   %tmp0 = and <3 x i8> %x, <i8 3, i8 undef, i8 3>

llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-sge-to-icmp-sle.ll

@@ -61,7 +61,7 @@ define <2 x i1> @p2_vec_nonsplat_edgecase(<2 x i8> %x) {
 
 define <3 x i1> @p3_vec_splat_undef(<3 x i8> %x) {
 ; CHECK-LABEL: @p3_vec_splat_undef(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp slt <3 x i8> [[X:%.*]], <i8 4, i8 undef, i8 4>
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp slt <3 x i8> [[X:%.*]], <i8 4, i8 4, i8 4>
 ; CHECK-NEXT:    ret <3 x i1> [[TMP1]]
 ;
   %tmp0 = and <3 x i8> %x, <i8 3, i8 undef, i8 3>

llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-sle-to-icmp-sle.ll

@@ -76,7 +76,7 @@ define <2 x i1> @p2_vec_nonsplat_edgecase() {
 define <3 x i1> @p3_vec_splat_undef() {
 ; CHECK-LABEL: @p3_vec_splat_undef(
 ; CHECK-NEXT:    [[X:%.*]] = call <3 x i8> @gen3x8()
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp slt <3 x i8> [[X]], <i8 4, i8 undef, i8 4>
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp slt <3 x i8> [[X]], <i8 4, i8 4, i8 4>
 ; CHECK-NEXT:    ret <3 x i1> [[TMP1]]
 ;
   %x = call <3 x i8> @gen3x8()

llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-uge-to-icmp-ule.ll

@@ -82,7 +82,7 @@ define <2 x i1> @p2_vec_nonsplat_edgecase1(<2 x i8> %x) {
 
 define <3 x i1> @p3_vec_splat_undef(<3 x i8> %x) {
 ; CHECK-LABEL: @p3_vec_splat_undef(
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult <3 x i8> [[X:%.*]], <i8 4, i8 undef, i8 4>
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult <3 x i8> [[X:%.*]], <i8 4, i8 4, i8 4>
 ; CHECK-NEXT:    ret <3 x i1> [[TMP1]]
 ;
   %tmp0 = and <3 x i8> %x, <i8 3, i8 undef, i8 3>

llvm/test/Transforms/InstCombine/canonicalize-constant-low-bit-mask-and-icmp-ule-to-icmp-ule.ll

@@ -99,7 +99,7 @@ define <2 x i1> @p2_vec_nonsplat_edgecase1() {
 define <3 x i1> @p3_vec_splat_undef() {
 ; CHECK-LABEL: @p3_vec_splat_undef(
 ; CHECK-NEXT:    [[X:%.*]] = call <3 x i8> @gen3x8()
-; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult <3 x i8> [[X]], <i8 4, i8 undef, i8 4>
+; CHECK-NEXT:    [[TMP1:%.*]] = icmp ult <3 x i8> [[X]], <i8 4, i8 4, i8 4>
 ; CHECK-NEXT:    ret <3 x i1> [[TMP1]]
 ;
   %x = call <3 x i8> @gen3x8()

llvm/test/Transforms/InstCombine/icmp-vec.ll

@@ -181,7 +181,7 @@ define <2 x i1> @PR27756_1(<2 x i8> %a) {
 
 define <3 x i1> @PR27756_2(<3 x i8> %a) {
 ; CHECK-LABEL: @PR27756_2(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp slt <3 x i8> [[A:%.*]], <i8 43, i8 undef, i8 1>
+; CHECK-NEXT:    [[CMP:%.*]] = icmp slt <3 x i8> [[A:%.*]], <i8 43, i8 43, i8 1>
 ; CHECK-NEXT:    ret <3 x i1> [[CMP]]
 ;
   %cmp = icmp sle <3 x i8> %a, <i8 42, i8 undef, i8 0>
@@ -190,7 +190,7 @@ define <3 x i1> @PR27756_2(<3 x i8> %a) {
 
 define <3 x i1> @PR27756_3(<3 x i8> %a) {
 ; CHECK-LABEL: @PR27756_3(
-; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <3 x i8> [[A:%.*]], <i8 undef, i8 0, i8 41>
+; CHECK-NEXT:    [[CMP:%.*]] = icmp sgt <3 x i8> [[A:%.*]], <i8 0, i8 0, i8 41>
 ; CHECK-NEXT:    ret <3 x i1> [[CMP]]
 ;
   %cmp = icmp sge <3 x i8> %a, <i8 undef, i8 1, i8 42>

llvm/test/Transforms/InstCombine/reuse-constant-from-select-in-icmp.ll

@@ -106,7 +106,7 @@ define <2 x i32> @p7_vec_splat_sgt(<2 x i32> %x, <2 x i32> %y) {
 
 define <2 x i32> @p8_vec_nonsplat_undef0(<2 x i32> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @p8_vec_nonsplat_undef0(
-; CHECK-NEXT:    [[T_INV:%.*]] = icmp ugt <2 x i32> [[X:%.*]], <i32 65535, i32 undef>
+; CHECK-NEXT:    [[T_INV:%.*]] = icmp ugt <2 x i32> [[X:%.*]], <i32 65535, i32 65535>
 ; CHECK-NEXT:    [[R:%.*]] = select <2 x i1> [[T_INV]], <2 x i32> <i32 65535, i32 65535>, <2 x i32> [[Y:%.*]]
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;
@@ -126,7 +126,7 @@ define <2 x i32> @p9_vec_nonsplat_undef1(<2 x i32> %x, <2 x i32> %y) {
 }
 define <2 x i32> @p10_vec_nonsplat_undef2(<2 x i32> %x, <2 x i32> %y) {
 ; CHECK-LABEL: @p10_vec_nonsplat_undef2(
-; CHECK-NEXT:    [[T_INV:%.*]] = icmp ugt <2 x i32> [[X:%.*]], <i32 65535, i32 undef>
+; CHECK-NEXT:    [[T_INV:%.*]] = icmp ugt <2 x i32> [[X:%.*]], <i32 65535, i32 65535>
 ; CHECK-NEXT:    [[R:%.*]] = select <2 x i1> [[T_INV]], <2 x i32> <i32 65535, i32 undef>, <2 x i32> [[Y:%.*]]
 ; CHECK-NEXT:    ret <2 x i32> [[R]]
 ;