[llvm] 4725158 - [SimplifyCFG] Cost required selects

[Sam Parker via llvm-commits]

Author: Sam Parker
Date: 2020-08-21T09:52:52+01:00
New Revision: 47251582f5c2ac5962f52bc6ddd3d612c56ab83d

URL: https://github.com/llvm/llvm-project/commit/47251582f5c2ac5962f52bc6ddd3d612c56ab83d
DIFF: https://github.com/llvm/llvm-project/commit/47251582f5c2ac5962f52bc6ddd3d612c56ab83d.diff

LOG: [SimplifyCFG] Cost required selects

Before we speculatively execute a basic block, query the cost of
inserting the necessary select instructions against the phi folding
threshold. For non-trivial insertions, a more accurate decision can
probably be made during machine if-conversion. With minsize we query
the CodeSize cost, otherwise we use SizeAndLatency.

Differential Revision: https://reviews.llvm.org/D82438

Added: 
    

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

Removed: 
    


################################################################################
diff  --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index 36161e3f1791..1f9e73f02cfd 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -2042,6 +2042,25 @@ bool SimplifyCFGOpt::SpeculativelyExecuteBB(BranchInst *BI, BasicBlock *ThenBB,
   BasicBlock *BB = BI->getParent();
   BasicBlock *EndBB = ThenBB->getTerminator()->getSuccessor(0);
 
+  TargetTransformInfo::TargetCostKind CostKind =
+    BI->getFunction()->hasMinSize() ?
+    TargetTransformInfo::TCK_CodeSize :
+    TargetTransformInfo::TCK_SizeAndLatency;
+  // 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,
+                                 CostKind);
+  }
+  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;

diff  --git a/llvm/test/Transforms/SimplifyCFG/ARM/select-costs.ll b/llvm/test/Transforms/SimplifyCFG/ARM/select-costs.ll
index c121061e0bea..5653493b3daa 100644
--- a/llvm/test/Transforms/SimplifyCFG/ARM/select-costs.ll
+++ b/llvm/test/Transforms/SimplifyCFG/ARM/select-costs.ll
@@ -6,6 +6,7 @@
 
 ; Test that the phis from for.inc.preheader aren't hoisted into sw.bb92 because
 ; the cost is too great - we can make a better decision later on.
+; FIXME: The Arm cost model needs to be updated for SizeAndLatency.
 define i32 @multiple_spec_select_costs(i8* %a, i32* %idx, i8 %in) {
 ; V8M-LABEL: @multiple_spec_select_costs(
 ; V8M-NEXT:  entry:
@@ -113,13 +114,13 @@ define i32 @multiple_spec_select_costs_minsize(i8* %a, i32* %idx, i8 %in) #0 {
 ; 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]] ]