Make the unreachable probability much much heavier. The previous

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

diff --git a/lib/Analysis/BranchProbabilityInfo.cpp b/lib/Analysis/BranchProbabilityInfo.cpp
index f9461c0b85385798f6f986909245de3a76e9dbc4..2730ce6c63bfe4b3cb9f650e9de8df50c74498f1 100644 (file)
--- a/lib/Analysis/BranchProbabilityInfo.cpp
+++ b/lib/Analysis/BranchProbabilityInfo.cpp
@@ -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;

diff --git a/test/Analysis/BranchProbabilityInfo/noreturn.ll b/test/Analysis/BranchProbabilityInfo/noreturn.ll
index c53c1ed113e8b11e8695fbbb3bc717c8a7680b06..8b9ae11f7d35b1943af55d9cd675e9fee659aeb9 100644 (file)
--- a/test/Analysis/BranchProbabilityInfo/noreturn.ll
+++ b/test/Analysis/BranchProbabilityInfo/noreturn.ll
@@ -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