[SCEV] Teach computeSCEVAtScope benefit from one-input Phi. PR39673

SCEV does not propagate arguments through one-input Phis so as to make it easy for the SCEV expander (and related code) to preserve LCSSA.  It's not entirely clear this restriction is neccessary, but for the moment it exists.   For this reason, we don't analyze single-entry phi inputs.  However it is possible that when an this input leaves the loop through LCSSA Phi, it is a provable constant.  Missing that results in an order of optimization issue in loop exit value rewriting where we miss some oppurtunities based on order in which we visit sibling loops.

This patch teaches computeSCEVAtScope about this case. We can generalize it later, but so far we can only replace LCSSA Phis with their constant loop-exiting values.  We should probably also add similiar logic directly in the SCEV construction path itself.

Patch by: mkazantsev (with revised commit message by me)
Differential Revision: https://reviews.llvm.org/D58113

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@363180 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/ScalarEvolution.cpp b/lib/Analysis/ScalarEvolution.cpp
index dc2d32856cc4dd7cecde2793c6c0ba6ee70345bb..88686427127974c24371d8cc48dbc673e7d39daa 100644 (file)
--- a/lib/Analysis/ScalarEvolution.cpp
+++ b/lib/Analysis/ScalarEvolution.cpp
@@ -8138,6 +8138,16 @@ const SCEV *ScalarEvolution::computeSCEVAtScope(const SCEV *V, const Loop *L) {
             if (RV) return getSCEV(RV);
           }
         }
+
+        // If there is a single-input Phi, evaluate it at our scope. If we can
+        // prove that this replacement does not break LCSSA form, use new value.
+        if (PN->getNumOperands() == 1) {
+          const SCEV *Input = getSCEV(PN->getOperand(0));
+          const SCEV *InputAtScope = getSCEVAtScope(Input, L);
+          // TODO: We can generalize it using LI.replacementPreservesLCSSAForm,
+          // for the simplest case just support constants.
+          if (isa<SCEVConstant>(InputAtScope)) return InputAtScope;
+        }
       }
 
       // Okay, this is an expression that we cannot symbolically evaluate

diff --git a/test/Transforms/IndVarSimplify/pr39673.ll b/test/Transforms/IndVarSimplify/pr39673.ll
index 4ab79a935c91eb1c8474e99ee6110a61da880165..422281a007af5e2372e57c19f5f0e5b3ad33d4ed 100644 (file)
--- a/test/Transforms/IndVarSimplify/pr39673.ll
+++ b/test/Transforms/IndVarSimplify/pr39673.ll
@@ -21,8 +21,7 @@ define i16 @test() {
 ; CHECK-NEXT:    [[CMP2:%.*]] = icmp ult i16 [[L2_ADD]], 2
 ; CHECK-NEXT:    br i1 [[CMP2]], label [[LOOP2]], label [[LOOP2_END:%.*]]
 ; CHECK:       loop2.end:
-; CHECK-NEXT:    [[K2_ADD_LCSSA:%.*]] = phi i16 [ [[K2_ADD]], [[LOOP2]] ]
-; CHECK-NEXT:    ret i16 [[K2_ADD_LCSSA]]
+; CHECK-NEXT:    ret i16 184
 ;
 entry:
   br label %loop1