[CSSPGO] Avoid deleting probe instruction in FoldValueComparisonIntoPredecessors

[wlei]

Summary

This change tries to fix a place missing moveAndDanglePseudoProbes . In FoldValueComparisonIntoPredecessors, it folds the BB into predecessors and then marked the BB unreachable. However, the original logic from the BB is still alive, deleting the probe will mislead the SampleLoader mark it as zero count sample.

[hoy]

Thanks for the fix. Is it possible to add a test?

[wlei]

Address Hongtao's feedback, add test case

[wlei]

In D102721#2767154, @hoy wrote:

Thanks for the fix. Is it possible to add a test?

Yes, test case is added.

[hoy]

LGTM, thanks.

As discussed offline, we may want to experiment treating all missing probes as dangling in the final pass of the pipeline and see how effective the counts inference can be. This would avoid such ad-hoc fixes including the previous ones we made.

[wenlei]

lgtm, thanks.

llvm/lib/Transforms/Utils/SimplifyCFG.cpp

if (DTU) {
     if (InfLoopBlock)
       Updates.push_back({DominatorTree::Insert, Pred, InfLoopBlock});
 
     Updates.push_back({DominatorTree::Delete, Pred, BB});
 
     DTU->applyUpdates(Updates);
   }
 
+  // Here the BB is not a dead block but folded into its predecessors, so move
+  // the probe and mark it as dangling.
+  moveAndDanglePseudoProbes(BB, NewSI);
+
   ++NumFoldValueComparisonIntoPredecessors;
   return true;
 }
 
 /// The specified terminator is a value equality comparison instruction
 /// (either a switch or a branch on "X == c").
 /// See if any of the predecessors of the terminator block are value comparisons
 /// on the same value.  If so, and if safe to do so, fold them together.

llvm/test/Transforms/SampleProfile/pseudo-probe-dangle3.ll

+; Test after FoldValueComparisonIntoPredecessors, one dangling probe is generated
+; RUN: opt -S -passes='pseudo-probe,simplifycfg' < %s | FileCheck %s
+
+
+; CHECK: if.end80:                                         ; preds = %if.end
+; CHECK-NEXT:   call void @llvm.pseudoprobe(i64 -2281696412744416103, i64 3, i32 0, i64 -1)
+; CHECK-NEXT:   call void @llvm.pseudoprobe(i64 -2281696412744416103, i64 4, i32 2, i64 -1)
+
+define dso_local i32 @readCBPandCoeffsFromNAL(i1 %c, i32 %x, i32 %y) local_unnamed_addr {
+;
+if.end:
+  br i1 %c, label %if.end80, label %if.then64
+
+if.then64:                                        ; preds = %if.end
+  ret i32 %y
+
+if.end80:                                         ; preds = %if.end
+  switch i32 %x, label %lor.lhs.false89 [
+  i32 10, label %if.end172237
+  i32 14, label %if.end172237
+  i32 9, label %if.end172
+  ]
+
+lor.lhs.false89:                                  ; preds = %lor.lhs.false89, %if.end80
+  %cmp91 = icmp eq i32 %x, 12
+  br i1 %cmp91, label %if.end172, label %lor.lhs.false89
+
+if.end172:                                        ; preds = %lor.lhs.false89, %if.end80
+  br label %if.end239
+
+if.end172237:                                     ; preds = %if.end80, %if.end80
+  br label %if.end239
+
+if.end239:                                        ; preds = %if.end172237, %if.end172
+  %cbp.0 = phi i32 [ 1, %if.end172237 ], [ 0, %if.end172 ]
+  ret i32 %cbp.0
+}