Fix invalid cast in instcombine UMul/ZExt idiom

[serge-sans-paille]

Summary

Fixes https://bugs.llvm.org/show_bug.cgi?id=25454

Do not assume IRBuilder creates Instruction where it can create Value.
Do not assume idiom operands are constant, leave generalisation ot the IRBuilder.

[sepavloff]

LGTM with small remark.

lib/Transforms/InstCombine/InstCombineCompares.cpp

3849

Explicit type instead of auto would improve readability.

[serge-sans-paille]

@sepavloff commited, thanks

lib/Transforms/InstCombine/InstCombineCompares.cpp

for (User *U : MulVal->users()) {
       if (TruncInst *TI = dyn_cast<TruncInst>(U)) {
         if (TI->getType()->getPrimitiveSizeInBits() == MulWidth)
           IC.replaceInstUsesWith(*TI, Mul);
         else
           TI->setOperand(0, Mul);
       } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(U)) {
         assert(BO->getOpcode() == Instruction::And);
         // Replace (mul & mask) --> zext (mul.with.overflow & short_mask)
-        ConstantInt *CI = cast<ConstantInt>(BO->getOperand(1));
-        APInt ShortMask = CI->getValue().trunc(MulWidth);
+        auto ShortMask = Builder->CreateTrunc(BO->getOperand(1),
+                                              Builder->getIntNTy(MulWidth));
         Value *ShortAnd = Builder->CreateAnd(Mul, ShortMask);
-        Instruction *Zext =
-            cast<Instruction>(Builder->CreateZExt(ShortAnd, BO->getType()));
-        IC.Worklist.Add(Zext);
+        Value *Zext = Builder->CreateZExt(ShortAnd, BO->getType());
+        if (auto *ZextI = dyn_cast<Instruction>(Zext))
+          IC.Worklist.Add(ZextI);
         IC.replaceInstUsesWith(*BO, Zext);
       } else {
         llvm_unreachable("Unexpected Binary operation");
       }
-      IC.Worklist.Add(cast<Instruction>(U));
+      if (auto *UI = dyn_cast<Instruction>(U))
+        IC.Worklist.Add(UI);
     }
   }
   if (isa<Instruction>(OtherVal))
     IC.Worklist.Add(cast<Instruction>(OtherVal));
 
   // The original icmp gets replaced with the overflow value, maybe inverted
   // depending on predicate.
   bool Inverse = false;

test/Transforms/InstCombine/2017-07-07-UMul-ZExt.ll

+; RUN: opt < %s -instcombine -S | FileCheck %s
+
+; CHECK: llvm.umul.with.overflow
+define i32 @sterix(i32, i8, i64) {
+entry:
+  %conv = zext i32 %0 to i64
+  %conv1 = sext i8 %1 to i32
+  %mul = mul i32 %conv1, 1945964878
+  %sh_prom = trunc i64 %2 to i32
+  %shr = lshr i32 %mul, %sh_prom
+  %conv2 = zext i32 %shr to i64
+  %mul3 = mul nuw nsw i64 %conv, %conv2
+  %conv6 = and i64 %mul3, 4294967295
+  %tobool = icmp ne i64 %conv6, %mul3
+  br i1 %tobool, label %lor.end, label %lor.rhs
+
+lor.rhs:
+  %and = and i64 %2, %mul3
+  %conv4 = trunc i64 %and to i32
+  %tobool7 = icmp ne i32 %conv4, 0
+  %lnot = xor i1 %tobool7, true
+  br label %lor.end
+
+lor.end:
+  %3 = phi i1 [ true, %entry ], [ %lnot, %lor.rhs ]
+  %conv8 = zext i1 %3 to i32
+  ret i32 %conv8
+}
+