[llvm] r193855 - Consider (x == -1) unlikely in BranchProbabilityInfo

[Hal Finkel]

Author: hfinkel
Date: Fri Nov  1 05:58:22 2013
New Revision: 193855

URL: http://llvm.org/viewvc/llvm-project?rev=193855&view=rev
Log:
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.

Modified:
    llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp
    llvm/trunk/test/Analysis/BranchProbabilityInfo/basic.ll

Modified: llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp?rev=193855&r1=193854&r2=193855&view=diff
==============================================================================
--- llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp (original)
+++ llvm/trunk/lib/Analysis/BranchProbabilityInfo.cpp Fri Nov  1 05:58:22 2013
@@ -398,10 +398,24 @@ bool BranchProbabilityInfo::calcZeroHeur
     // 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;
   }

Modified: llvm/trunk/test/Analysis/BranchProbabilityInfo/basic.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Analysis/BranchProbabilityInfo/basic.ll?rev=193855&r1=193854&r2=193855&view=diff
==============================================================================
--- llvm/trunk/test/Analysis/BranchProbabilityInfo/basic.ll (original)
+++ llvm/trunk/test/Analysis/BranchProbabilityInfo/basic.ll Fri Nov  1 05:58:22 2013
@@ -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
+}
+