[NewGVN] Fix handling of assumes

[nikic]

Summary

This patch fixes two bugs:

• test1: Previously assume(a >= 5) concluded that a == 5. That's only valid for assume(a == 5)...
• test2: If operands were swapped, additional users were added to the wrong cmp operand. This resulted in an "unsettled iteration" assertion failure.

[• dberlin]

Thanks!

[nikic]

I don't have commit access, could someone please commit this patch?

[fhahn]

Can do, either later today or sometime tomorrow

lib/Transforms/Scalar/NewGVN.cpp

NewGVN::performSymbolicPredicateInfoEvaluation(Instruction *I) const {
   if (shouldSwapOperands(FirstOp, SecondOp)) {
     std::swap(FirstOp, SecondOp);
     SwappedOps = true;
   }
   CmpInst::Predicate Predicate =
       SwappedOps ? Cmp->getSwappedPredicate() : Cmp->getPredicate();
 
   if (isa<PredicateAssume>(PI)) {
-    // If the comparison is true when the operands are equal, then we know the
-    // operands are equal, because assumes must always be true.
-    if (CmpInst::isTrueWhenEqual(Predicate)) {
+    // If we assume the operands are equal, then they are equal.
+    if (Predicate == CmpInst::ICMP_EQ) {
       addPredicateUsers(PI, I);
-      addAdditionalUsers(Cmp->getOperand(0), I);
+      addAdditionalUsers(SwappedOps ? Cmp->getOperand(1) : Cmp->getOperand(0),
+                         I);
       return createVariableOrConstant(FirstOp);
     }
   }
   if (const auto *PBranch = dyn_cast<PredicateBranch>(PI)) {
     // If we are *not* a copy of the comparison, we may equal to the other
     // operand when the predicate implies something about equality of
     // operations.  In particular, if the comparison is true/false when the
     // operands are equal, and we are on the right edge, we know this operation

test/Transforms/NewGVN/assumes.ll

+; RUN: opt < %s -newgvn -S | FileCheck %s
+
+; CHECK-LABEL: @test1
+; CHECK: ret i32 %arg
+define i32 @test1(i32 %arg) {
+  %cmp = icmp sge i32 %arg, 5
+  call void @llvm.assume(i1 %cmp)
+  ret i32 %arg
+}
+
+; CHECK-LABEL: @test2
+; CHECK: ret i32 %arg
+define i32 @test2(i32 %arg, i1 %b) {
+  br label %bb
+
+bb:
+  %a = phi i32 [ 1, %0 ], [ 2, %bb ]
+  %cmp = icmp eq i32 %arg, %a
+  call void @llvm.assume(i1 %cmp)
+  br i1 %b, label %bb, label %end
+
+end:
+  ret i32 %arg
+}
+
+declare void @llvm.assume(i1 %cond)