[InstCombine] Prevent the transform of the comparison for all undef elements

[Allen]

Summary

Some pass such as Loop Vectorization(-fvectorize) may generate undef elements
constant, which will ball out the combine optimization.
Note: Part undef elements don't prevent the transform of the comparison.

[paulwalker-arm]

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

129

Verify the case when all elements are undefined.

Not sure if I've missed some detail but when I run this test using an upstream pull from this morning I achieve the same output you're matching against here without needing this patch?

[Allen]

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

129

Oh， sorry, my bad. I work with old commit dd644ddf8 before, and now the new upstream seem work fine, and don't need such work around

llvm/lib/Transforms/InstCombine/InstCombineCompares.cpp

for (unsigned i = 0; i != NumElts; ++i) {
       if (!SafeReplacementConstant)
         SafeReplacementConstant = CI;
     }
   } else {
     // ConstantExpr?
     return llvm::None;
   }
 
+  // It may be all elements are under value.
+  if (C->containsUndefOrPoisonElement() && SafeReplacementConstant == nullptr)
+    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.
   // TODO: in case of poison, it is safe; let's replace undefs only.
   if (C->containsUndefOrPoisonElement()) {
-    assert(SafeReplacementConstant && "Replacement constant not set");
     C = Constant::replaceUndefsWith(C, SafeReplacementConstant);
   }
 
   CmpInst::Predicate NewPred = CmpInst::getFlippedStrictnessPredicate(Pred);
 
   // Increment or decrement the constant.
   Constant *OneOrNegOne = ConstantInt::get(Type, WillIncrement ? 1 : -1, true);
   Constant *NewC = ConstantExpr::getAdd(C, OneOrNegOne);

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

 ; CHECK-NEXT:    [[CMP:%.*]] = icmp slt i9 [[TMP1]], 1
 ; CHECK-NEXT:    ret i1 [[CMP]]
 ;
   %splat = shufflevector <2 x i9> %v, <2 x i9> poison, <3 x i32> zeroinitializer
   %cast = bitcast <3 x i9> %splat to i27
   %cmp = icmp slt i27 %cast, 262657 ; 0x040201
   ret i1 %cmp
 }
+
+; Verify the case that all elements are under define
+define <4 x i1> @test_i32_ule_pattern(i32 %val) {
+; CHECK-LABEL: @test_i32_ule_pattern(
+; CHECK-NEXT:    ret <4 x i1> <i1 true, i1 true, i1 true, i1 true>
+;
+  %insvec = insertelement <4 x i32> poison, i32 %val, i32 0
+  %vec = shufflevector <4 x i32> %insvec, <4 x i32> poison, <4 x i32> zeroinitializer
+  %cond = icmp ule <4 x i32> %vec, <i32 undef, i32 poison, i32 poison, i32 poison>
+  ret <4 x i1> %cond
+}