Do not set branch weight if the branch weight annotation is present.

[danielcdh]

Summary

ThinLTO will annotate the CFG twice. If the branch weight is set by the first annotation, we should not set the branch weight again in the second annotation because the first annotation is more accurate as there is less optimization that could affect debug info accuracy.

[tejohnson]

LGTM

lib/Transforms/IPO/SampleProfile.cpp

for (unsigned I = 0; I < TI->getNumSuccessors(); ++I) {
       if (Weight != 0) {
         if (Weight > MaxWeight) {
           MaxWeight = Weight;
           MaxDestLoc = Succ->getFirstNonPHIOrDbgOrLifetime()->getDebugLoc();
         }
       }
     }
 
+    uint64_t TempWeight;
     // Only set weights if there is at least one non-zero weight.
     // In any other case, let the analyzer set weights.
-    if (MaxWeight > 0) {
+    // Do not set weights if the weights are present. In ThinLTO, the profile
+    // annotation is done twice. If the first annotation already set the
+    // weights, the second pass does not need to set it.
+    if (MaxWeight > 0 && !TI->extractProfTotalWeight(TempWeight)) {
       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() + ":" +

test/Transforms/SampleProfile/branch.ll

if.then.2:                                        ; preds = %if.end
   call void @llvm.dbg.declare(metadata i32* %u, metadata !42, metadata !17), !dbg !44
   store i32 0, i32* %u, align 4, !dbg !44
   br label %for.cond, !dbg !45
 
 for.cond:                                         ; preds = %for.inc, %if.then.2
   %6 = load i32, i32* %u, align 4, !dbg !46
   %7 = load i32, i32* %limit, align 4, !dbg !48
   %cmp5 = icmp slt i32 %6, %7, !dbg !49
-  br i1 %cmp5, label %for.body, label %for.end, !dbg !50
+  br i1 %cmp5, label %for.body, label %for.end, !dbg !50, !prof !80
+; CHECK: edge for.cond -> for.body probability is 0x73333333 / 0x80000000 = 90.00%
+; CHECK: edge for.cond -> for.end probability is 0x0ccccccd / 0x80000000 = 10.00%
 
 for.body:                                         ; preds = %for.cond
   call void @llvm.dbg.declare(metadata double* %x, metadata !51, metadata !17), !dbg !53
   %8 = load double, double* %s, align 8, !dbg !54
   store double %8, double* %x, align 8, !dbg !53
   %9 = load double, double* %x, align 8, !dbg !55
   %add = fadd double %9, 3.049000e+00, !dbg !56
   %10 = load i32, i32* %u, align 4, !dbg !57

 !72 = !DILocation(line: 18, column: 20, scope: !73)
 !73 = distinct !DILexicalBlock(scope: !33, file: !1, line: 17, column: 11)
 !74 = !DILocation(line: 18, column: 15, scope: !73)
 !75 = !DILocation(line: 18, column: 13, scope: !73)
 !76 = !DILocation(line: 20, column: 30, scope: !6)
 !77 = !DILocation(line: 20, column: 4, scope: !6)
 !78 = !DILocation(line: 21, column: 4, scope: !6)
 !79 = !DILocation(line: 22, column: 2, scope: !6)
+!80 = !{!"branch_weights", i32 90, i32 10}