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.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@193855 91177308-0d34-0410-b5e6-96231b3b80d8

diff --git a/lib/Analysis/BranchProbabilityInfo.cpp b/lib/Analysis/BranchProbabilityInfo.cpp
index 7cdf828c2f4b42694c839b069493867da2f0b7dd..86560ca33d0c8d4c86316670aaec8db2ab71aa88 100644 (file)
--- a/lib/Analysis/BranchProbabilityInfo.cpp
+++ b/lib/Analysis/BranchProbabilityInfo.cpp
@@ -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;
   }

diff --git a/test/Analysis/BranchProbabilityInfo/basic.ll b/test/Analysis/BranchProbabilityInfo/basic.ll
index c6e1bde81dffab8f12055f8be240e886a5ac5b9a..05cb31dca0ee9bbf08b4dcd2c9ecfa93f2056c35 100644 (file)
--- a/test/Analysis/BranchProbabilityInfo/basic.ll
+++ b/test/Analysis/BranchProbabilityInfo/basic.ll
@@ -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
+}
+