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[SimplifyCFG] Fix a cost modeling oversight in branch commoning 

The cost modeling was not accounting for the fact we were duplicating the instruction once per predecessor.  With a default threshold of 1, this meant we were actually creating #pred copies.

Adding to the fun, there is *absolutely no* test coverage for this.  Simply bailing for more than one predecessor passes all checked in tests.

llvm-svn: 341001
This commit is contained in:
Philip Reames 2018-08-30 00:03:02 +00:00
parent 32a8a2028c
commit 6bd16b5850
2 changed files with 60 additions and 2 deletions
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10
llvm/lib/Transforms/Utils/SimplifyCFG.cpp
View File

@ -2539,6 +2539,8 @@ static bool extractPredSuccWeights(BranchInst *PBI, BranchInst *BI,
bool llvm::FoldBranchToCommonDest(BranchInst *BI, unsigned BonusInstThreshold) {
BasicBlock *BB = BI->getParent();
const unsigned PredCount = pred_size(BB);
Instruction *Cond = nullptr;
if (BI->isConditional())
Cond = dyn_cast<Instruction>(BI->getCondition());
@ -2588,7 +2590,8 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, unsigned BonusInstThreshold) {
// too many instructions and these involved instructions can be executed
// unconditionally. We denote all involved instructions except the condition
// as "bonus instructions", and only allow this transformation when the
// number of the bonus instructions does not exceed a certain threshold.
// number of the bonus instructions we'll need to create when cloning into
// each predecessor does not exceed a certain threshold.
unsigned NumBonusInsts = 0;
for (auto I = BB->begin(); Cond != &*I; ++I) {
// Ignore dbg intrinsics.
@ -2603,7 +2606,10 @@ bool llvm::FoldBranchToCommonDest(BranchInst *BI, unsigned BonusInstThreshold) {
// I is used in the same BB. Since BI uses Cond and doesn't have more slots
// to use any other instruction, User must be an instruction between next(I)
// and Cond.
++NumBonusInsts;
// Account for the cost of duplicating this instruction into each
// predecessor.
NumBonusInsts += PredCount;
// Early exits once we reach the limit.
if (NumBonusInsts > BonusInstThreshold)
return false;

52
llvm/test/Transforms/SimplifyCFG/branch-fold-threshold.ll
View File

@ -1,5 +1,6 @@
; RUN: opt %s -simplifycfg -S | FileCheck %s --check-prefix=NORMAL
; RUN: opt %s -simplifycfg -S -bonus-inst-threshold=2 | FileCheck %s --check-prefix=AGGRESSIVE
; RUN: opt %s -simplifycfg -S -bonus-inst-threshold=4 | FileCheck %s --check-prefix=WAYAGGRESSIVE
define i32 @foo(i32 %a, i32 %b, i32 %c, i32 %d, i32* %input) {
; NORMAL-LABEL: @foo(
@ -26,3 +27,54 @@ cond.end:
%cond = phi i32 [ %0, %cond.false ], [ 0, %lor.lhs.false ], [ 0, %entry ]
ret i32 %cond
}
declare void @distinct_a();
declare void @distinct_b();
;; Like foo, but have to duplicate into multiple predecessors
define i32 @bar(i32 %a, i32 %b, i32 %c, i32 %d, i32* %input) {
; NORMAL-LABEL: @bar(
; AGGRESSIVE-LABEL: @bar(
entry:
%cmp_split = icmp slt i32 %d, %b
%cmp = icmp sgt i32 %d, 3
br i1 %cmp_split, label %pred_a, label %pred_b
pred_a:
; NORMAL-LABEL: pred_a:
; AGGRESSIVE-LABEL: pred_a:
; WAYAGGRESSIVE-LABEL: pred_a:
; NORMAL: br i1
; AGGRESSIVE: br i1
; WAYAGGRESSIVE: br i1
call void @distinct_a();
br i1 %cmp, label %cond.end, label %lor.lhs.false
pred_b:
; NORMAL-LABEL: pred_b:
; AGGRESSIVE-LABEL: pred_b:
; WAYAGGRESSIVE-LABEL: pred_b:
; NORMAL: br i1
; AGGRESSIVE: br i1
; WAYAGGRESSIVE: br i1
call void @distinct_b();
br i1 %cmp, label %cond.end, label %lor.lhs.false
lor.lhs.false:
%mul = shl i32 %c, 1
%add = add nsw i32 %mul, %a
%cmp1 = icmp slt i32 %add, %b
br i1 %cmp1, label %cond.false, label %cond.end
; NORMAL-LABEL: lor.lhs.false:
; AGGRESIVE-LABEL: lor.lhs.false:
; WAYAGGRESIVE-LABEL: lor.lhs.false:
; NORMAL: br i1
; AGGRESSIVE: br i1
; WAYAGGRESSIVE-NOT: br i1
cond.false:
%0 = load i32, i32* %input, align 4
br label %cond.end
cond.end:
%cond = phi i32 [ %0, %cond.false ], [ 0, %lor.lhs.false ],[ 0, %pred_a ],[ 0, %pred_b ]
ret i32 %cond
}