Only set branch weight during sample pgo annotation when max_weight of the branch is non-zero. Otherwise use default static profile to set branch probability.

[danielcdh]

Summary

It does not make sense to set equal weights for all unkown branches as we have static branch prediction available.

[dnovillo]

LGTM.

lib/Transforms/IPO/SampleProfile.cpp

for (unsigned I = 0; I < TI->getNumSuccessors(); ++I) {
           MaxWeight = Weight;
           MaxDestLoc = Succ->getFirstNonPHIOrDbgOrLifetime()->getDebugLoc();
         }
       }
     }
 
     // Only set weights if there is at least one non-zero weight.
     // In any other case, let the analyzer set weights.
+    if (MaxWeight > 0) {
       DEBUG(dbgs() << "SUCCESS. Found non-zero weights.\n");
       TI->setMetadata(llvm::LLVMContext::MD_prof,
                       MDB.createBranchWeights(Weights));
       DebugLoc BranchLoc = TI->getDebugLoc();
       emitOptimizationRemark(
           Ctx, DEBUG_TYPE, F, MaxDestLoc,
           Twine("most popular destination for conditional branches at ") +
               ((BranchLoc) ? Twine(BranchLoc->getFilename() + ":" +
                                    Twine(BranchLoc.getLine()) + ":" +
                                    Twine(BranchLoc.getCol()))
                            : Twine("<UNKNOWN LOCATION>")));
+    } else {
+      DEBUG(dbgs() << "SKIPPED. All branch weights are zero.\n");
+    }
   }
 }
 
 /// \brief Get the line number for the function header.
 ///
 /// This looks up function \p F in the current compilation unit and
 /// retrieves the line number where the function is defined. This is
 /// line 0 for all the samples read from the profile file. Every line

test/Transforms/SampleProfile/fnptr.ll

 ; The two profiles used in this test are the same but encoded in different
 ; formats. This checks that we produce the same profile annotations regardless
 ; of the profile format.
 ;
 ; RUN: opt < %s -sample-profile -sample-profile-file=%S/Inputs/fnptr.prof | opt -analyze -branch-prob | FileCheck %s
 ; RUN: opt < %s -sample-profile -sample-profile-file=%S/Inputs/fnptr.binprof | opt -analyze -branch-prob | FileCheck %s
 
 ; RUN: opt < %s -passes=sample-profile -sample-profile-file=%S/Inputs/fnptr.prof | opt -analyze -branch-prob | FileCheck %s
 ; RUN: opt < %s -passes=sample-profile -sample-profile-file=%S/Inputs/fnptr.binprof | opt -analyze -branch-prob | FileCheck %s
 
 ; CHECK:   edge for.body3 -> if.then probability is 0x1a56a56a / 0x80000000 = 20.58%
 ; CHECK:   edge for.body3 -> if.else probability is 0x65a95a96 / 0x80000000 = 79.42%
 ; CHECK:   edge for.inc -> for.inc12 probability is 0x000fdc50 / 0x80000000 = 0.05%
 ; CHECK:   edge for.inc -> for.body3 probability is 0x7ff023b0 / 0x80000000 = 99.95%
-; CHECK:   edge for.inc12 -> for.end14 probability is 0x40000000 / 0x80000000 = 50.00%
-; CHECK:   edge for.inc12 -> for.cond1.preheader probability is 0x40000000 / 0x80000000 = 50.00%
+; CHECK:   edge for.inc12 -> for.end14 probability is 0x04000000 / 0x80000000 = 3.12%
+; CHECK:   edge for.inc12 -> for.cond1.preheader probability is 0x7c000000 / 0x80000000 = 96.88%
 
 ; Original C++ test case.
 ;
 ; #include <stdlib.h>
 ; #include <math.h>
 ; #include <stdio.h>
 ;
 ; #define N 10000
for.body3:                                        ; preds = %for.inc, %for.cond1.preheader
   %S.122 = phi double [ %S.024, %for.cond1.preheader ], [ %S.2, %for.inc ]
   %call = tail call i32 @rand() #3, !dbg !15
   %rem = srem i32 %call, 100, !dbg !15
   %cmp4 = icmp slt i32 %rem, 30, !dbg !15
   %_Z3fooi._Z3bari = select i1 %cmp4, double (i32)* @_Z3fooi, double (i32)* @_Z3bari, !dbg !15
   %call5 = tail call i32 @rand() #3, !dbg !16
   %rem6 = srem i32 %call5, 100, !dbg !16
   %cmp7 = icmp slt i32 %rem6, 10, !dbg !16
-  br i1 %cmp7, label %if.then, label %if.else, !dbg !16, !prof !17
+  br i1 %cmp7, label %if.then, label %if.else, !dbg !16
 
 if.then:                                          ; preds = %for.body3
   %mul = mul nsw i32 %j.023, 300, !dbg !18
   %add = add nsw i32 %mul, %i.025, !dbg !18
   %call8 = tail call double %_Z3fooi._Z3bari(i32 %add), !dbg !18
   br label %for.inc, !dbg !18
 
 if.else:                                          ; preds = %for.body3
   %div = sdiv i32 %j.023, 840, !dbg !19
   %sub = sub nsw i32 %i.025, %div, !dbg !19
   %call10 = tail call double %_Z3fooi._Z3bari(i32 %sub), !dbg !19
   br label %for.inc
 
 for.inc:                                          ; preds = %if.then, %if.else
   %call8.pn = phi double [ %call8, %if.then ], [ %call10, %if.else ]
   %S.2 = fadd double %S.122, %call8.pn, !dbg !18
   %inc = add nsw i32 %j.023, 1, !dbg !20
   %exitcond = icmp eq i32 %j.023, 5999, !dbg !14
-  br i1 %exitcond, label %for.inc12, label %for.body3, !dbg !14, !prof !21
+  br i1 %exitcond, label %for.inc12, label %for.body3, !dbg !14
 
 for.inc12:                                        ; preds = %for.inc
   %S.2.lcssa = phi double [ %S.2, %for.inc ]
   %inc13 = add nsw i32 %i.025, 1, !dbg !22
   %exitcond26 = icmp eq i32 %i.025, 9999, !dbg !12
-  br i1 %exitcond26, label %for.end14, label %for.cond1.preheader, !dbg !12, !prof !23
+  br i1 %exitcond26, label %for.end14, label %for.cond1.preheader, !dbg !12
 
 for.end14:                                        ; preds = %for.inc12
   %S.2.lcssa.lcssa = phi double [ %S.2.lcssa, %for.inc12 ]
   %call15 = tail call i32 (i8*, ...) @printf(i8* getelementptr inbounds ([9 x i8], [9 x i8]* @.str, i64 0, i64 0), double %S.2.lcssa.lcssa), !dbg !24
   ret i32 0, !dbg !25
 }
 
 ; Function Attrs: nounwind

 !9 = !DILocation(line: 13, column: 3, scope: !10)
 !10 = distinct !DISubprogram(name: "bar", line: 12, isLocal: false, isDefinition: true, flags: DIFlagPrototyped, isOptimized: true, unit: !26, scopeLine: 12, file: !4, scope: !5, type: !6, variables: !7)
 !11 = !DILocation(line: 13, column: 14, scope: !10)
 !12 = !DILocation(line: 19, column: 3, scope: !13)
 !13 = distinct !DISubprogram(name: "main", line: 16, isLocal: false, isDefinition: true, flags: DIFlagPrototyped, isOptimized: true, unit: !26, scopeLine: 16, file: !4, scope: !5, type: !6, variables: !7)
 !14 = !DILocation(line: 20, column: 5, scope: !13)
 !15 = !DILocation(line: 21, column: 15, scope: !13)
 !16 = !DILocation(line: 22, column: 11, scope: !13)
-!17 = !{!"branch_weights", i32 534, i32 2064}
 !18 = !DILocation(line: 23, column: 14, scope: !13)
 !19 = !DILocation(line: 25, column: 14, scope: !13)
 !20 = !DILocation(line: 20, column: 28, scope: !13)
-!21 = !{!"branch_weights", i32 0, i32 1075}
 !22 = !DILocation(line: 19, column: 26, scope: !13)
-!23 = !{!"branch_weights", i32 0, i32 534}
 !24 = !DILocation(line: 27, column: 3, scope: !13)
 !25 = !DILocation(line: 28, column: 3, scope: !13)
 !26 = distinct !DICompileUnit(language: DW_LANG_C_plus_plus, producer: "clang version 3.5 ", isOptimized: false, emissionKind: FullDebug, file: !4)