[PATCH] D82438: [SimplifyCFG] Cost required selects

[Sam Parker via Phabricator via llvm-commits]

samparker updated this revision to Diff 278749.
samparker added a comment.

Rebased after precommitting the test.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D82438/new/

https://reviews.llvm.org/D82438

Files:
  llvm/lib/Transforms/Utils/SimplifyCFG.cpp
  llvm/test/Transforms/SimplifyCFG/ARM/select-costs.ll


Index: llvm/test/Transforms/SimplifyCFG/ARM/select-costs.ll
===================================================================
--- llvm/test/Transforms/SimplifyCFG/ARM/select-costs.ll
+++ llvm/test/Transforms/SimplifyCFG/ARM/select-costs.ll
@@ -16,13 +16,13 @@
 ; V8M:       sw.bb92:
 ; V8M-NEXT:    [[C_OFF_I150:%.*]] = add i8 [[IN]], -48
 ; V8M-NEXT:    [[UGT_9:%.*]] = icmp ugt i8 [[C_OFF_I150]], 9
-; V8M-NEXT:    [[SPEC_SELECT:%.*]] = select i1 [[UGT_9]], i1 false, i1 true
-; V8M-NEXT:    [[SPEC_SELECT1:%.*]] = select i1 [[UGT_9]], i32 1, i32 7
+; V8M-NEXT:    br i1 [[UGT_9]], label [[FOR_INC_PREHEADER]], label [[SELECT_UNFOLD198:%.*]]
+; V8M:       select.unfold198:
 ; V8M-NEXT:    br label [[FOR_INC_PREHEADER]]
 ; V8M:       for.inc.preheader:
-; V8M-NEXT:    [[STR_PH_0:%.*]] = phi i8* [ [[GEP_A_2]], [[ENTRY:%.*]] ], [ [[INCDEC_PTR109_C4]], [[SW_BB92]] ]
-; V8M-NEXT:    [[CMP:%.*]] = phi i1 [ false, [[ENTRY]] ], [ [[SPEC_SELECT]], [[SW_BB92]] ]
-; V8M-NEXT:    [[PHI_RES:%.*]] = phi i32 [ 1, [[ENTRY]] ], [ [[SPEC_SELECT1]], [[SW_BB92]] ]
+; V8M-NEXT:    [[STR_PH_0:%.*]] = phi i8* [ [[INCDEC_PTR109_C4]], [[SELECT_UNFOLD198]] ], [ [[INCDEC_PTR109_C4]], [[SW_BB92]] ], [ [[GEP_A_2]], [[ENTRY:%.*]] ]
+; V8M-NEXT:    [[CMP:%.*]] = phi i1 [ true, [[SELECT_UNFOLD198]] ], [ false, [[SW_BB92]] ], [ false, [[ENTRY]] ]
+; V8M-NEXT:    [[PHI_RES:%.*]] = phi i32 [ 7, [[SELECT_UNFOLD198]] ], [ 1, [[SW_BB92]] ], [ 1, [[ENTRY]] ]
 ; V8M-NEXT:    br label [[FOR_INC:%.*]]
 ; V8M:       for.inc:
 ; V8M-NEXT:    [[STR_PH_1:%.*]] = phi i8* [ [[INCDEC_PTR109:%.*]], [[FOR_BODY:%.*]] ], [ [[STR_PH_0]], [[FOR_INC_PREHEADER]] ]
Index: llvm/lib/Transforms/Utils/SimplifyCFG.cpp
===================================================================
--- llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2036,6 +2036,21 @@
   BasicBlock *BB = BI->getParent();
   BasicBlock *EndBB = ThenBB->getTerminator()->getSuccessor(0);
 
+  // Check how expensive it will be to insert the necessary selects.
+  unsigned CostOfSelects = 0;
+  for (PHINode &PN : EndBB->phis()) {
+    unsigned OrigI = PN.getBasicBlockIndex(BB);
+    unsigned ThenI = PN.getBasicBlockIndex(ThenBB);
+    Value *OrigV = PN.getIncomingValue(OrigI);
+    Value *ThenV = PN.getIncomingValue(ThenI);
+    if (OrigV != ThenV)
+      CostOfSelects +=
+          TTI.getCmpSelInstrCost(Instruction::Select, PN.getType(), nullptr,
+                                 TargetTransformInfo::TCK_SizeAndLatency);
+  }
+  if (CostOfSelects > PHINodeFoldingThreshold * TargetTransformInfo::TCC_Basic)
+    return false;
+
   // If ThenBB is actually on the false edge of the conditional branch, remember
   // to swap the select operands later.
   bool Invert = false;


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