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Make the unreachable probability much much heavier. The previous

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probability wouldn't be considered "hot" in some weird loop structures
or other compounding probability patterns. This makes it much harder to
confuse, but isn't really a principled fix. I'd actually like it if we
could model a zero probability, as it would make this much easier to
reason about. Suggestions for how to do this better are welcome.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@147142 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Chandler Carruth 2011-12-22 09:26:37 +00:00
parent 3963d617b3
commit 51f40a725b
2 changed files with 12 additions and 11 deletions
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5
lib/Analysis/BranchProbabilityInfo.cpp
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@ -65,8 +65,9 @@ static const uint32_t UR_TAKEN_WEIGHT = 1;
///
/// This is the weight for a branch not being taken toward a block that
/// terminates (eventually) in unreachable. Such a branch is essentially never
/// taken.
static const uint32_t UR_NONTAKEN_WEIGHT = 1023;
/// taken. Set the weight to an absurdly high value so that nested loops don't
/// easily subsume it.
static const uint32_t UR_NONTAKEN_WEIGHT = 1024*1024 - 1;
static const uint32_t PH_TAKEN_WEIGHT = 20;
static const uint32_t PH_NONTAKEN_WEIGHT = 12;

18
test/Analysis/BranchProbabilityInfo/noreturn.ll
View File

@ -8,8 +8,8 @@ define i32 @test1(i32 %a, i32 %b) {
entry:
%cond = icmp eq i32 %a, 42
br i1 %cond, label %exit, label %abort
; CHECK: edge entry -> exit probability is 1023 / 1024
; CHECK: edge entry -> abort probability is 1 / 1024
; CHECK: edge entry -> exit probability is 1048575 / 1048576
; CHECK: edge entry -> abort probability is 1 / 1048576
abort:
call void @abort() noreturn
@ -26,11 +26,11 @@ entry:
i32 2, label %case_b
i32 3, label %case_c
i32 4, label %case_d]
; CHECK: edge entry -> exit probability is 1023 / 1027
; CHECK: edge entry -> case_a probability is 1 / 1027
; CHECK: edge entry -> case_b probability is 1 / 1027
; CHECK: edge entry -> case_c probability is 1 / 1027
; CHECK: edge entry -> case_d probability is 1 / 1027
; CHECK: edge entry -> exit probability is 1048575 / 1048579
; CHECK: edge entry -> case_a probability is 1 / 1048579
; CHECK: edge entry -> case_b probability is 1 / 1048579
; CHECK: edge entry -> case_c probability is 1 / 1048579
; CHECK: edge entry -> case_d probability is 1 / 1048579
case_a:
br label %case_b
@ -55,8 +55,8 @@ define i32 @test3(i32 %a, i32 %b) {
entry:
%cond1 = icmp eq i32 %a, 42
br i1 %cond1, label %exit, label %dom
; CHECK: edge entry -> exit probability is 1023 / 1024
; CHECK: edge entry -> dom probability is 1 / 1024
; CHECK: edge entry -> exit probability is 1048575 / 1048576
; CHECK: edge entry -> dom probability is 1 / 1048576
dom:
%cond2 = icmp ult i32 %a, 42