[llvm] r309072 - [SCEV] Remove unnecessary call to forgetMemoizedResults

[Sanjoy Das via llvm-commits]

Author: sanjoy
Date: Tue Jul 25 18:32:19 2017
New Revision: 309072

URL: http://llvm.org/viewvc/llvm-project?rev=309072&view=rev
Log:
[SCEV] Remove unnecessary call to forgetMemoizedResults

`SCEVUnknown::allUsesReplacedWith` does not need to call `forgetMemoizedResults`
since RAUW does a value-equivalent replacement by assumption.  If this
assumption was false then the later setValPtr(New) call would be incorrect too.

This is a non-trivial performance optimization for functions with a large number
of loops since `forgetMemoizedResults` walks all loop backedge taken counts to
see if any of them use the SCEVUnknown being RAUWed.  However, this improvement
is difficult to demonstrate without checking in an excessively large IR file.

Modified:
    llvm/trunk/lib/Analysis/ScalarEvolution.cpp

Modified: llvm/trunk/lib/Analysis/ScalarEvolution.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/ScalarEvolution.cpp?rev=309072&r1=309071&r2=309072&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/ScalarEvolution.cpp (original)
+++ llvm/trunk/lib/Analysis/ScalarEvolution.cpp Tue Jul 25 18:32:19 2017
@@ -415,9 +415,6 @@ void SCEVUnknown::deleted() {
 }
 
 void SCEVUnknown::allUsesReplacedWith(Value *New) {
-  // Clear this SCEVUnknown from various maps.
-  SE->forgetMemoizedResults(this);
-
   // Remove this SCEVUnknown from the uniquing map.
   SE->UniqueSCEVs.RemoveNode(this);