[LoopRotate] Set loop back edge weight to not less than exit weight (#86496)

Branch weight from sample-based PGO may be not inaccurate due to
sampling. If the loop body must be executed, then origin loop back
edge weight must be not less than exit weight.

Author: HaohaiWen

llvm/lib/Transforms/Utils/LoopRotationUtils.cpp

@@ -347,9 +347,19 @@ static void updateBranchWeights(BranchInst &PreHeaderBI, BranchInst &LoopBI,
         // probabilities as if there are only 0-trip and 1-trip cases.
         ExitWeight0 = OrigLoopExitWeight - OrigLoopBackedgeWeight;
       }
+    } else {
+      // Theoretically, if the loop body must be executed at least once, the
+      // backedge count must be not less than exit count. However the branch
+      // weight collected by sampling-based PGO may be not very accurate due to
+      // sampling. Therefore this workaround is required here to avoid underflow
+      // of unsigned in following update of branch weight.
+      if (OrigLoopExitWeight > OrigLoopBackedgeWeight)
+        OrigLoopBackedgeWeight = OrigLoopExitWeight;
     }
+    assert(OrigLoopExitWeight >= ExitWeight0 && "Bad branch weight");
     ExitWeight1 = OrigLoopExitWeight - ExitWeight0;
     EnterWeight = ExitWeight1;
+    assert(OrigLoopBackedgeWeight >= EnterWeight && "Bad branch weight");
     LoopBackWeight = OrigLoopBackedgeWeight - EnterWeight;
   } else if (OrigLoopExitWeight == 0) {
     if (OrigLoopBackedgeWeight == 0) {

llvm/test/Transforms/LoopRotate/update-branch-weights.ll

@@ -240,7 +240,7 @@ loop_exit:
 
 ; BFI_AFTER-LABEL: block-frequency-info: func6_inaccurate_branch_weight
 ; BFI_AFTER: - entry: {{.*}} count = 1024
-; BFI_AFTER: - loop_body: {{.*}} count = 4294967296
+; BFI_AFTER: - loop_body: {{.*}} count = 1024
 ; BFI_AFTER: - loop_exit: {{.*}} count = 1024
 
 ; IR-LABEL: define void @func6_inaccurate_branch_weight(
@@ -292,4 +292,4 @@ loop_exit:
 ; IR: [[PROF_FUNC3_0]] = !{!"branch_weights", i32 0, i32 1}
 ; IR: [[PROF_FUNC4_0]] = !{!"branch_weights", i32 1, i32 0}
 ; IR: [[PROF_FUNC5_0]] = !{!"branch_weights", i32 0, i32 0}
-; IR: [[PROF_FUNC6_0]] = !{!"branch_weights", i32 -1, i32 1024}
+; IR: [[PROF_FUNC6_0]] = !{!"branch_weights", i32 0, i32 1024}