[llvm-diff] Fix false alarm on PHI node

[Nuullll]

Summary

This addresses https://github.com/llvm/llvm-project/issues/56076

llvm-diff was reporting a false alarm on the %res PHI node of the following module, comparing with itself by running llvm-diff a.ll a.ll:

define double @foo() {
entry:
  br i1 true, label %else, label %exit

else:
  %0 = extractelement <2 x double> zeroinitializer, i64 0
  br label %exit

exit:
  %res = phi double [ 0.000000e+00, %entry ], [ %0, %else ]
  ret double 0.000000e+00
}

The reason was that:

• The exit block is processed before the else block.
• When comparing against the PHI node, its operand %0 is not visited yet. So the fake difference is reported because %0 is neither mapped nor tentative.

Fixed by calling diff() on instructions when checking the operand equivalence.

Signed-off-by: Yilong Guo <[e-mail]>

[fhahn]

Thanks for the patch. Some comments inline

llvm/test/tools/llvm-diff/phinode-1.ll

2

I think this also needs a FileCheck call to check that llvm-diff handles the case as expected? Please see other tests like llvm/test/tools/llvm-diff/anon-func.ll for example.

13

might be good to also add a test case with operands of the phi flipped.

llvm/tools/llvm-diff/lib/DifferenceEngine.cpp

594

Does this handle cycles in phi nodes correctly? It would be good to at least have a couple of tests with cycles.

[Nuullll]

llvm/tools/llvm-diff/lib/DifferenceEngine.cpp

594

Thanks a lot for reviewing!
This actually is causing infinite recursions for cycled phi nodes.
I'll reconsider the algorithm.

llvm/test/tools/llvm-diff/phinode-1.ll

+; Issue: https://github.com/llvm/llvm-project/issues/56076
+; RUN: llvm-diff %s %s
+
+define double @foo() {
+entry:
+  br i1 true, label %else, label %exit
+
+else:
+  %0 = extractelement <2 x double> zeroinitializer, i64 0
+  br label %exit
+
+exit:
+  %res = phi double [ 0.000000e+00, %entry ], [ %0, %else ]
+  ret double 0.000000e+00
+}

llvm/tools/llvm-diff/lib/DifferenceEngine.cpp

bool equivalentAsOperands(const Value *L, const Value *R) {
 
     // Value subtypes:  Argument, Constant, Instruction, BasicBlock,
     //                  InlineAsm, MDNode, MDString, PseudoSourceValue
 
     if (isa<Constant>(L))
       return equivalentAsOperands(cast<Constant>(L), cast<Constant>(R));
 
     if (isa<Instruction>(L))
-      return Values[L] == R || TentativeValues.count(std::make_pair(L, R));
+      return Values[L] == R || TentativeValues.count(std::make_pair(L, R)) ||
+             !diff(cast<Instruction>(L), cast<Instruction>(R), false, false);
 
     if (isa<Argument>(L))
       return Values[L] == R;
 
     if (isa<BasicBlock>(L))
       return Blocks[cast<BasicBlock>(L)] != R;
 
     // Pretend everything else is identical.