Remove threading of Xor over selects and phis, with an explanation
of why such threading is pointless.
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119798 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/lib/Analysis/InstructionSimplify.cpp b/lib/Analysis/InstructionSimplify.cpp
index 7848d50..97612f4 100644
--- a/lib/Analysis/InstructionSimplify.cpp
+++ b/lib/Analysis/InstructionSimplify.cpp
@@ -250,6 +250,16 @@
}
// FIXME: Could pull several more out of instcombine.
+
+ // Threading Add over selects and phi nodes is pointless, so don't bother.
+ // Threading over the select in "A + select(cond, B, C)" means evaluating
+ // "A+B" and "A+C" and seeing if they are equal; but they are equal if and
+ // only if B and C are equal. If B and C are equal then (since we assume
+ // that operands have already been simplified) "select(cond, B, C)" should
+ // have been simplified to the common value of B and C already. Analysing
+ // "A+B" and "A+C" thus gains nothing, but costs compile time. Similarly
+ // for threading over phi nodes.
+
return 0;
}
@@ -454,19 +464,14 @@
(A == Op0 || B == Op0))
return A == Op0 ? B : A;
- // If the operation is with the result of a select instruction, check whether
- // operating on either branch of the select always yields the same value.
- if (MaxRecurse && (isa<SelectInst>(Op0) || isa<SelectInst>(Op1)))
- if (Value *V = ThreadBinOpOverSelect(Instruction::Xor, Op0, Op1, TD, DT,
- MaxRecurse-1))
- return V;
-
- // If the operation is with the result of a phi instruction, check whether
- // operating on all incoming values of the phi always yields the same value.
- if (MaxRecurse && (isa<PHINode>(Op0) || isa<PHINode>(Op1)))
- if (Value *V = ThreadBinOpOverPHI(Instruction::Xor, Op0, Op1, TD, DT,
- MaxRecurse-1))
- return V;
+ // Threading Xor over selects and phi nodes is pointless, so don't bother.
+ // Threading over the select in "A ^ select(cond, B, C)" means evaluating
+ // "A^B" and "A^C" and seeing if they are equal; but they are equal if and
+ // only if B and C are equal. If B and C are equal then (since we assume
+ // that operands have already been simplified) "select(cond, B, C)" should
+ // have been simplified to the common value of B and C already. Analysing
+ // "A^B" and "A^C" thus gains nothing, but costs compile time. Similarly
+ // for threading over phi nodes.
return 0;
}