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Consider (x == -1) unlikely in BranchProbabilityInfo 

This adds another heuristic to BPI, similar to the existing heuristic that
considers (x == 0) unlikely to be true. As suggested in the PACT'98 paper by
Deitrich, Cheng, and Hwu, -1 is often used to indicate an invalid index, and
equality comparisons with -1 are also unlikely to succeed. Local
experimentation supports this hypothesis: This yields a 1-2% speedup in the
test-suite sqlite benchmark on the PPC A2 core, with no significant
regressions.

llvm-svn: 193855
This commit is contained in:
Hal Finkel 2013-11-01 10:58:22 +00:00
parent 1d1debf126
commit 4d94930bcb
2 changed files with 57 additions and 4 deletions
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22
llvm/lib/Analysis/BranchProbabilityInfo.cpp
View File

@ -398,10 +398,24 @@ bool BranchProbabilityInfo::calcZeroHeuristics(BasicBlock *BB) {
// InstCombine canonicalizes X <= 0 into X < 1.
// X <= 0 -> Unlikely
isProb = false;
} else if (CV->isAllOnesValue() && CI->getPredicate() == CmpInst::ICMP_SGT) {
// InstCombine canonicalizes X >= 0 into X > -1.
// X >= 0 -> Likely
isProb = true;
} else if (CV->isAllOnesValue()) {
switch (CI->getPredicate()) {
case CmpInst::ICMP_EQ:
// X == -1 -> Unlikely
isProb = false;
break;
case CmpInst::ICMP_NE:
// X != -1 -> Likely
isProb = true;
break;
case CmpInst::ICMP_SGT:
// InstCombine canonicalizes X >= 0 into X > -1.
// X >= 0 -> Likely
isProb = true;
break;
default:
return false;
}
} else {
return false;
}

39
llvm/test/Analysis/BranchProbabilityInfo/basic.ll
View File

@ -173,3 +173,42 @@ exit:
%ret = phi i32 [ %val4, %then ], [ %val3, %else ]
ret i32 %ret
}
define i32 @zero1(i32 %i, i32 %a, i32 %b) {
; CHECK: Printing analysis {{.*}} for function 'zero1'
entry:
%cond = icmp eq i32 %i, 0
br i1 %cond, label %then, label %else
; CHECK: edge entry -> then probability is 12 / 32
; CHECK: edge entry -> else probability is 20 / 32
then:
br label %exit
else:
br label %exit
exit:
%result = phi i32 [ %a, %then ], [ %b, %else ]
ret i32 %result
}
define i32 @zero2(i32 %i, i32 %a, i32 %b) {
; CHECK: Printing analysis {{.*}} for function 'zero2'
entry:
%cond = icmp ne i32 %i, -1
br i1 %cond, label %then, label %else
; CHECK: edge entry -> then probability is 20 / 32
; CHECK: edge entry -> else probability is 12 / 32
then:
br label %exit
else:
br label %exit
exit:
%result = phi i32 [ %a, %then ], [ %b, %else ]
ret i32 %result
}