[SimplifyCFG] Probabilities associated with same condition are constant #155734
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mtrofin
requested review from
david-xl,
nikic,
jdenny-ornl,
alanzhao1 and
rotateright
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@llvm/pr-subscribers-llvm-transforms Author: Mircea Trofin (mtrofin) ChangesThe branch weights capture probability. The probability has everything to do with the (SSA) value the condition is predicated on, and nothing to do with the position in the CFG. Full diff: https://github.com/llvm/llvm-project/pull/155734.diff 2 Files Affected:
diff --git a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
index ef110a6922f05..2ef44c6d2849c 100644
--- a/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
+++ b/llvm/lib/Transforms/Utils/SimplifyCFG.cpp
@@ -4808,23 +4808,12 @@ static bool SimplifyCondBranchToCondBranch(BranchInst *PBI, BranchInst *BI,
SelectInst *NV = cast<SelectInst>(
Builder.CreateSelect(PBICond, PBIV, BIV, PBIV->getName() + ".mux"));
PN.setIncomingValue(PBBIdx, NV);
- // Although the select has the same condition as PBI, the original branch
- // weights for PBI do not apply to the new select because the select's
- // 'logical' edges are incoming edges of the phi that is eliminated, not
- // the outgoing edges of PBI.
+ // The select has the same condition as PBI. The probabilities don't
+ // change.
if (HasWeights) {
- uint64_t PredCommon = PBIOp ? PredFalseWeight : PredTrueWeight;
- uint64_t PredOther = PBIOp ? PredTrueWeight : PredFalseWeight;
- uint64_t SuccCommon = BIOp ? SuccFalseWeight : SuccTrueWeight;
- uint64_t SuccOther = BIOp ? SuccTrueWeight : SuccFalseWeight;
- // The weight to PredCommonDest should be PredCommon * SuccTotal.
- // The weight to PredOtherDest should be PredOther * SuccCommon.
- uint64_t NewWeights[2] = {PredCommon * (SuccCommon + SuccOther),
- PredOther * SuccCommon};
-
- fitWeights(NewWeights);
-
- setBranchWeights(NV, NewWeights[0], NewWeights[1],
+ uint64_t TrueWeight = PBIOp ? PredFalseWeight : PredTrueWeight;
+ uint64_t FalseWeight = PBIOp ? PredTrueWeight : PredFalseWeight;
+ setBranchWeights(NV, TrueWeight, FalseWeight,
/*IsExpected=*/false);
}
}
diff --git a/llvm/test/Transforms/SimplifyCFG/preserve-branchweights.ll b/llvm/test/Transforms/SimplifyCFG/preserve-branchweights.ll
index 0f78e236b4248..f200de7503816 100644
--- a/llvm/test/Transforms/SimplifyCFG/preserve-branchweights.ll
+++ b/llvm/test/Transforms/SimplifyCFG/preserve-branchweights.ll
@@ -610,8 +610,8 @@ define i32 @SimplifyCondBranchToCondBranchSwapMissingWeight(i1 %cmpa, i1 %cmpb)
; CHECK-NEXT: [[CMPA_NOT:%.*]] = xor i1 [[CMPA:%.*]], true
; CHECK-NEXT: [[CMPB_NOT:%.*]] = xor i1 [[CMPB:%.*]], true
; CHECK-NEXT: [[BRMERGE:%.*]] = select i1 [[CMPA_NOT]], i1 true, i1 [[CMPB_NOT]]
-; CHECK-NEXT: [[DOTMUX:%.*]] = select i1 [[CMPA_NOT]], i32 0, i32 2, !prof [[PROF17:![0-9]+]]
-; CHECK-NEXT: [[OUTVAL:%.*]] = select i1 [[BRMERGE]], i32 [[DOTMUX]], i32 1, !prof [[PROF18:![0-9]+]]
+; CHECK-NEXT: [[DOTMUX:%.*]] = select i1 [[CMPA_NOT]], i32 0, i32 2, !prof [[PROF15]]
+; CHECK-NEXT: [[OUTVAL:%.*]] = select i1 [[BRMERGE]], i32 [[DOTMUX]], i32 1, !prof [[PROF17:![0-9]+]]
; CHECK-NEXT: ret i32 [[OUTVAL]]
;
block1:
@@ -636,7 +636,7 @@ define void @or_icmps_harmful(i32 %x, i32 %y, ptr %p) {
; CHECK-LABEL: @or_icmps_harmful(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[EXPECTED_TRUE:%.*]] = icmp sgt i32 [[X:%.*]], -1
-; CHECK-NEXT: br i1 [[EXPECTED_TRUE]], label [[EXIT:%.*]], label [[RARE:%.*]], !prof [[PROF19:![0-9]+]]
+; CHECK-NEXT: br i1 [[EXPECTED_TRUE]], label [[EXIT:%.*]], label [[RARE:%.*]], !prof [[PROF18:![0-9]+]]
; CHECK: rare:
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: br i1 [[EXPENSIVE]], label [[EXIT]], label [[FALSE:%.*]]
@@ -669,7 +669,7 @@ define void @or_icmps_harmful_inverted(i32 %x, i32 %y, ptr %p) {
; CHECK-LABEL: @or_icmps_harmful_inverted(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[EXPECTED_FALSE:%.*]] = icmp sgt i32 [[X:%.*]], -1
-; CHECK-NEXT: br i1 [[EXPECTED_FALSE]], label [[RARE:%.*]], label [[EXIT:%.*]], !prof [[PROF20:![0-9]+]]
+; CHECK-NEXT: br i1 [[EXPECTED_FALSE]], label [[RARE:%.*]], label [[EXIT:%.*]], !prof [[PROF19:![0-9]+]]
; CHECK: rare:
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: br i1 [[EXPENSIVE]], label [[EXIT]], label [[FALSE:%.*]]
@@ -702,7 +702,7 @@ define void @or_icmps_probably_not_harmful(i32 %x, i32 %y, ptr %p) {
; CHECK-NEXT: [[EXPECTED_TRUE:%.*]] = icmp sgt i32 [[X:%.*]], -1
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: [[OR_COND:%.*]] = select i1 [[EXPECTED_TRUE]], i1 true, i1 [[EXPENSIVE]]
-; CHECK-NEXT: br i1 [[OR_COND]], label [[EXIT:%.*]], label [[FALSE:%.*]], !prof [[PROF21:![0-9]+]], !unpredictable [[META22:![0-9]+]]
+; CHECK-NEXT: br i1 [[OR_COND]], label [[EXIT:%.*]], label [[FALSE:%.*]], !prof [[PROF20:![0-9]+]], !unpredictable [[META21:![0-9]+]]
; CHECK: false:
; CHECK-NEXT: store i8 42, ptr [[P:%.*]], align 1
; CHECK-NEXT: br label [[EXIT]]
@@ -734,7 +734,7 @@ define void @or_icmps_not_that_harmful(i32 %x, i32 %y, ptr %p) {
; CHECK-NEXT: [[EXPECTED_TRUE:%.*]] = icmp sgt i32 [[X:%.*]], -1
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: [[OR_COND:%.*]] = select i1 [[EXPECTED_TRUE]], i1 true, i1 [[EXPENSIVE]]
-; CHECK-NEXT: br i1 [[OR_COND]], label [[EXIT:%.*]], label [[FALSE:%.*]], !prof [[PROF23:![0-9]+]]
+; CHECK-NEXT: br i1 [[OR_COND]], label [[EXIT:%.*]], label [[FALSE:%.*]], !prof [[PROF22:![0-9]+]]
; CHECK: false:
; CHECK-NEXT: store i8 42, ptr [[P:%.*]], align 1
; CHECK-NEXT: br label [[EXIT]]
@@ -766,7 +766,7 @@ define void @or_icmps_not_that_harmful_inverted(i32 %x, i32 %y, ptr %p) {
; CHECK-NEXT: [[EXPECTED_TRUE:%.*]] = icmp sgt i32 [[X:%.*]], -1
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: [[OR_COND:%.*]] = select i1 [[EXPECTED_TRUE]], i1 true, i1 [[EXPENSIVE]]
-; CHECK-NEXT: br i1 [[OR_COND]], label [[EXIT:%.*]], label [[FALSE:%.*]], !prof [[PROF24:![0-9]+]]
+; CHECK-NEXT: br i1 [[OR_COND]], label [[EXIT:%.*]], label [[FALSE:%.*]], !prof [[PROF23:![0-9]+]]
; CHECK: false:
; CHECK-NEXT: store i8 42, ptr [[P:%.*]], align 1
; CHECK-NEXT: br label [[EXIT]]
@@ -797,7 +797,7 @@ define void @or_icmps_useful(i32 %x, i32 %y, ptr %p) {
; CHECK-NEXT: [[EXPECTED_TRUE:%.*]] = icmp sle i32 [[X:%.*]], -1
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: [[OR_COND:%.*]] = select i1 [[EXPECTED_TRUE]], i1 true, i1 [[EXPENSIVE]]
-; CHECK-NEXT: br i1 [[OR_COND]], label [[EXIT:%.*]], label [[FALSE:%.*]], !prof [[PROF25:![0-9]+]]
+; CHECK-NEXT: br i1 [[OR_COND]], label [[EXIT:%.*]], label [[FALSE:%.*]], !prof [[PROF24:![0-9]+]]
; CHECK: false:
; CHECK-NEXT: store i8 42, ptr [[P:%.*]], align 1
; CHECK-NEXT: br label [[EXIT]]
@@ -828,7 +828,7 @@ define void @or_icmps_useful_inverted(i32 %x, i32 %y, ptr %p) {
; CHECK-NEXT: [[EXPECTED_FALSE:%.*]] = icmp sgt i32 [[X:%.*]], -1
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: [[OR_COND:%.*]] = select i1 [[EXPECTED_FALSE]], i1 true, i1 [[EXPENSIVE]]
-; CHECK-NEXT: br i1 [[OR_COND]], label [[EXIT:%.*]], label [[FALSE:%.*]], !prof [[PROF25]]
+; CHECK-NEXT: br i1 [[OR_COND]], label [[EXIT:%.*]], label [[FALSE:%.*]], !prof [[PROF24]]
; CHECK: false:
; CHECK-NEXT: store i8 42, ptr [[P:%.*]], align 1
; CHECK-NEXT: br label [[EXIT]]
@@ -889,7 +889,7 @@ define void @and_icmps_harmful(i32 %x, i32 %y, ptr %p) {
; CHECK-LABEL: @and_icmps_harmful(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[EXPECTED_FALSE:%.*]] = icmp sgt i32 [[X:%.*]], -1
-; CHECK-NEXT: br i1 [[EXPECTED_FALSE]], label [[RARE:%.*]], label [[EXIT:%.*]], !prof [[PROF20]]
+; CHECK-NEXT: br i1 [[EXPECTED_FALSE]], label [[RARE:%.*]], label [[EXIT:%.*]], !prof [[PROF19]]
; CHECK: rare:
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: br i1 [[EXPENSIVE]], label [[FALSE:%.*]], label [[EXIT]]
@@ -922,7 +922,7 @@ define void @and_icmps_harmful_inverted(i32 %x, i32 %y, ptr %p) {
; CHECK-LABEL: @and_icmps_harmful_inverted(
; CHECK-NEXT: entry:
; CHECK-NEXT: [[EXPECTED_TRUE:%.*]] = icmp sgt i32 [[X:%.*]], -1
-; CHECK-NEXT: br i1 [[EXPECTED_TRUE]], label [[EXIT:%.*]], label [[RARE:%.*]], !prof [[PROF19]]
+; CHECK-NEXT: br i1 [[EXPECTED_TRUE]], label [[EXIT:%.*]], label [[RARE:%.*]], !prof [[PROF18]]
; CHECK: rare:
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: br i1 [[EXPENSIVE]], label [[FALSE:%.*]], label [[EXIT]]
@@ -957,7 +957,7 @@ define void @and_icmps_not_that_harmful(i32 %x, i32 %y, ptr %p) {
; CHECK-NEXT: [[EXPECTED_FALSE:%.*]] = icmp sgt i32 [[X:%.*]], -1
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: [[OR_COND:%.*]] = select i1 [[EXPECTED_FALSE]], i1 [[EXPENSIVE]], i1 false
-; CHECK-NEXT: br i1 [[OR_COND]], label [[FALSE:%.*]], label [[EXIT:%.*]], !prof [[PROF26:![0-9]+]]
+; CHECK-NEXT: br i1 [[OR_COND]], label [[FALSE:%.*]], label [[EXIT:%.*]], !prof [[PROF25:![0-9]+]]
; CHECK: false:
; CHECK-NEXT: store i8 42, ptr [[P:%.*]], align 1
; CHECK-NEXT: br label [[EXIT]]
@@ -989,7 +989,7 @@ define void @and_icmps_not_that_harmful_inverted(i32 %x, i32 %y, ptr %p) {
; CHECK-NEXT: [[EXPECTED_TRUE:%.*]] = icmp sle i32 [[X:%.*]], -1
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: [[OR_COND:%.*]] = select i1 [[EXPECTED_TRUE]], i1 [[EXPENSIVE]], i1 false
-; CHECK-NEXT: br i1 [[OR_COND]], label [[FALSE:%.*]], label [[EXIT:%.*]], !prof [[PROF26]]
+; CHECK-NEXT: br i1 [[OR_COND]], label [[FALSE:%.*]], label [[EXIT:%.*]], !prof [[PROF25]]
; CHECK: false:
; CHECK-NEXT: store i8 42, ptr [[P:%.*]], align 1
; CHECK-NEXT: br label [[EXIT]]
@@ -1020,7 +1020,7 @@ define void @and_icmps_useful(i32 %x, i32 %y, ptr %p) {
; CHECK-NEXT: [[EXPECTED_TRUE:%.*]] = icmp sgt i32 [[X:%.*]], -1
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: [[OR_COND:%.*]] = select i1 [[EXPECTED_TRUE]], i1 [[EXPENSIVE]], i1 false
-; CHECK-NEXT: br i1 [[OR_COND]], label [[FALSE:%.*]], label [[EXIT:%.*]], !prof [[PROF27:![0-9]+]]
+; CHECK-NEXT: br i1 [[OR_COND]], label [[FALSE:%.*]], label [[EXIT:%.*]], !prof [[PROF26:![0-9]+]]
; CHECK: false:
; CHECK-NEXT: store i8 42, ptr [[P:%.*]], align 1
; CHECK-NEXT: br label [[EXIT]]
@@ -1051,7 +1051,7 @@ define void @and_icmps_useful_inverted(i32 %x, i32 %y, ptr %p) {
; CHECK-NEXT: [[EXPECTED_FALSE:%.*]] = icmp sle i32 [[X:%.*]], -1
; CHECK-NEXT: [[EXPENSIVE:%.*]] = icmp eq i32 [[Y:%.*]], 0
; CHECK-NEXT: [[OR_COND:%.*]] = select i1 [[EXPECTED_FALSE]], i1 [[EXPENSIVE]], i1 false
-; CHECK-NEXT: br i1 [[OR_COND]], label [[FALSE:%.*]], label [[EXIT:%.*]], !prof [[PROF27]]
+; CHECK-NEXT: br i1 [[OR_COND]], label [[FALSE:%.*]], label [[EXIT:%.*]], !prof [[PROF26]]
; CHECK: false:
; CHECK-NEXT: store i8 42, ptr [[P:%.*]], align 1
; CHECK-NEXT: br label [[EXIT]]
@@ -1114,19 +1114,18 @@ exit:
; CHECK: [[PROF10]] = !{!"branch_weights", i32 8, i32 33}
; CHECK: [[PROF11]] = !{!"branch_weights", i32 112017436, i32 -735157296}
; CHECK: [[PROF12]] = !{!"branch_weights", i32 3, i32 5}
-; CHECK: [[PROF13]] = !{!"branch_weights", i32 22, i32 12}
+; CHECK: [[PROF13]] = !{!"branch_weights", i32 2, i32 3}
; CHECK: [[PROF14]] = !{!"branch_weights", i32 34, i32 21}
-; CHECK: [[PROF15]] = !{!"branch_weights", i32 33, i32 14}
+; CHECK: [[PROF15]] = !{!"branch_weights", i32 3, i32 2}
; CHECK: [[PROF16]] = !{!"branch_weights", i32 47, i32 8}
-; CHECK: [[PROF17]] = !{!"branch_weights", i32 6, i32 2}
-; CHECK: [[PROF18]] = !{!"branch_weights", i32 8, i32 2}
-; CHECK: [[PROF19]] = !{!"branch_weights", i32 99, i32 1}
-; CHECK: [[PROF20]] = !{!"branch_weights", i32 1, i32 99}
-; CHECK: [[PROF21]] = !{!"branch_weights", i32 199, i32 1}
-; CHECK: [[META22]] = !{}
-; CHECK: [[PROF23]] = !{!"branch_weights", i32 197, i32 1}
-; CHECK: [[PROF24]] = !{!"branch_weights", i32 100, i32 98}
-; CHECK: [[PROF25]] = !{!"branch_weights", i32 101, i32 99}
-; CHECK: [[PROF26]] = !{!"branch_weights", i32 1, i32 197}
-; CHECK: [[PROF27]] = !{!"branch_weights", i32 99, i32 101}
+; CHECK: [[PROF17]] = !{!"branch_weights", i32 8, i32 2}
+; CHECK: [[PROF18]] = !{!"branch_weights", i32 99, i32 1}
+; CHECK: [[PROF19]] = !{!"branch_weights", i32 1, i32 99}
+; CHECK: [[PROF20]] = !{!"branch_weights", i32 199, i32 1}
+; CHECK: [[META21]] = !{}
+; CHECK: [[PROF22]] = !{!"branch_weights", i32 197, i32 1}
+; CHECK: [[PROF23]] = !{!"branch_weights", i32 100, i32 98}
+; CHECK: [[PROF24]] = !{!"branch_weights", i32 101, i32 99}
+; CHECK: [[PROF25]] = !{!"branch_weights", i32 1, i32 197}
+; CHECK: [[PROF26]] = !{!"branch_weights", i32 99, i32 101}
;.
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The branch weights capture probability. The probability has everything to do with the (SSA) value the condition is predicated on, and nothing to do with the position in the CFG.