[PGO] Fix insane counts due to nonreturn calls

[xur]

Summary

Since we don't break BBs for function calls. We might get some insane counts
(wrap of unsigned) in the presence of noreturn calls.

This patch sets these counts to zero instead of the wrapped number.

[davidxl]

lib/Transforms/Instrumentation/PGOInstrumentation.cpp

888

Can you explain how this happens? For non return calls, should an edge be added from the call's BB to exit bb?

test/Transforms/PGOProfile/noreturncall.ll

9

exit is not used .

[xur]

Using the test case as the example: exit() is called in bar0. The instrumentation is in bb2 (which calls bar0), bb4 and bb5.
bb2 is 21. bb4 is 0 and bb5 is 20. To make the edge flow balanced, we have to make edge 1-->3 as -1.

The real issue is that we don't have an edge from bar0 to the fake exit. We don't know if bar0 has a noreturn call.
The real solution is to split the BB with calls and and a fake edge.

We know this limitation when implementing IR PGO and we chose to tolerate this.

[davidxl]

lgtm

lib/Transforms/Instrumentation/PGOInstrumentation.cpp

889

Add a comment here that "If the one of the successor block can early terminate (no-return), we can end up with situation where out edge sum count is larger as the source BB's count is collected by a post-dominated block'.

lib/Transforms/Instrumentation/PGOInstrumentation.cpp

for (auto &BB : reverse(F)) {
         } else if (Count->UnknownCountInEdge == 0) {
           Count->CountValue = sumEdgeCount(Count->InEdges);
           Count->CountValid = true;
           Changes = true;
         }
       }
       if (Count->CountValid) {
         if (Count->UnknownCountOutEdge == 1) {
-          uint64_t Total = Count->CountValue - sumEdgeCount(Count->OutEdges);
+          uint64_t Total = 0;
+          uint64_t OutSum = sumEdgeCount(Count->OutEdges);
+          if (Count->CountValue > OutSum)
+            Total = Count->CountValue - OutSum;
           setEdgeCount(Count->OutEdges, Total);
           Changes = true;
         }
         if (Count->UnknownCountInEdge == 1) {
-          uint64_t Total = Count->CountValue - sumEdgeCount(Count->InEdges);
+          uint64_t Total = 0;
+          uint64_t InSum = sumEdgeCount(Count->InEdges);
+          if (Count->CountValue > InSum)
+            Total = Count->CountValue - InSum;
           setEdgeCount(Count->InEdges, Total);
           Changes = true;
         }
       }
     }
   }
 
   DEBUG(dbgs() << "Populate counts in " << NumPasses << " passes.\n");

test/Transforms/PGOProfile/Inputs/noreturncall.proftext

+# IR level Instrumentation Flag
+:ir
+foo
+# Func Hash:
[phone]
+# Num Counters:
+3
+# Counter Values:
+20
+21
+0

test/Transforms/PGOProfile/noreturncall.ll

+; RUN: llvm-profdata merge %S/Inputs/noreturncall.proftext -o %t.profdata
+; RUN: opt < %s -pgo-instr-use -pgo-test-profile-file=%t.profdata -S -debug-only=pgo-instrumentation 2>&1 | FileCheck %s --check-prefix=USE
+; RUN: opt < %s -passes=pgo-instr-use -pgo-test-profile-file=%t.profdata -S -debug-only=pgo-instrumentation 2>&1 | FileCheck %s --check-prefix=USE
+
+target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
+target triple = "x86_64-unknown-linux-gnu"
+
+declare i32 @bar0(i32)
+declare void @exit(i32)
+
+define i32 @bar2(i32 %i) {
+entry:
+  unreachable
+}
+
+define i32 @foo(i32 %i, i32 %j, i32 %k) {
+entry:
+  %cmp = icmp slt i32 %i, 999
+  br i1 %cmp, label %if.then, label %if.end
+
+if.then:
+  %call = call i32 @bar0(i32 %i)
+  br label %if.end
+
+if.end:
+  %ret.0 = phi i32 [ %call, %if.then ], [ 0, %entry ]
+  %cmp1 = icmp sgt i32 %j, 1000
+  %cmp3 = icmp sgt i32 %k, 99
+  %or.cond = and i1 %cmp1, %cmp3
+  br i1 %or.cond, label %if.then4, label %if.end7
+
+if.then4:
+  %call5 = call i32 @bar2(i32 undef)
+  br label %if.end7
+
+if.end7:
+  %mul = mul nsw i32 %ret.0, %ret.0
+  ret i32 %mul
+}
+; USE:  Edge 0: 1-->3  c  W=8000  Count=0
+; USE:  Edge 1: 3-->5  c  W=8000  Count=20
+; USE:  Edge 2: 0-->1     W=16  Count=21
+; USE:  Edge 3: 5-->0 *   W=16  Count=20
+; USE:  Edge 4: 1-->2     W=8  Count=21
+; USE:  Edge 5: 2-->3 *   W=8  Count=21
+; USE:  Edge 6: 3-->4     W=8  Count=0
+; USE:  Edge 7: 4-->5 *   W=8  Count=0