[JTS] Correctly handle all zero profile values in VP metadata (#193402)

We can end up with cases where the VP metadata only has zero profile
values, for example if all of the functions end up being external and
uninstrumented. This caused fixes an assertion failure on the BOLT
builder that came up last time we tried to turn the pass on by default.

Author: boomanaiden154

llvm/lib/Transforms/Scalar/JumpTableToSwitch.cpp

@@ -193,9 +193,11 @@ expandToSwitch(CallBase *CB, const JumpTableTy &JT, DomTreeUpdater &DTU,
     return OptimizationRemark(DEBUG_TYPE, "ReplacedJumpTableWithSwitch", CB)
            << "expanded indirect call into switch";
   });
-  if (HadProfile && !ProfcheckDisableMetadataFixes) {
-    // At least one of the targets must've been taken.
-    assert(llvm::any_of(BranchWeights, not_equal_to(0)));
+  // Only set branch weights on the switch if we have non-zero branch weights.
+  // We can have no non-zero branch weights while having VP metadata if for
+  // example, all of the functions are external and not instrumented.
+  if (HadProfile && !ProfcheckDisableMetadataFixes &&
+      llvm::any_of(BranchWeights, not_equal_to(0))) {
     setBranchWeights(*Switch, downscaleWeights(BranchWeights),
                      /*IsExpected=*/false);
   } else

llvm/test/Transforms/JumpTableToSwitch/profile-no-guid-metadata.ll

@@ -82,14 +82,45 @@ define i32 @caller_dup_guid(i32 %idx) {
   ret i32 %r
 }
 
+define i32 @caller_only_zero_guids(i32 %idx) {
+; CHECK-LABEL: define i32 @caller_only_zero_guids(
+; CHECK-SAME: i32 [[IDX:%.*]]) {
+; CHECK-NEXT:    [[GEP:%.*]] = getelementptr inbounds [2 x ptr], ptr @jt, i32 0, i32 [[IDX]]
+; CHECK-NEXT:    [[FPTR:%.*]] = load ptr, ptr [[GEP]], align 8
+; CHECK-NEXT:    switch i32 [[IDX]], label %[[DEFAULT_SWITCH_CASE_UNREACHABLE:.*]] [
+; CHECK-NEXT:      i32 0, label %[[CALL_0:.*]]
+; CHECK-NEXT:      i32 1, label %[[CALL_1:.*]]
+; CHECK-NEXT:    ], !prof [[PROF2:![0-9]+]]
+; CHECK:       [[DEFAULT_SWITCH_CASE_UNREACHABLE]]:
+; CHECK-NEXT:    unreachable
+; CHECK:       [[CALL_0]]:
+; CHECK-NEXT:    [[TMP1:%.*]] = call i32 @jt_target_0()
+; CHECK-NEXT:    br [[DOTTAIL:label %.*]]
+; CHECK:       [[CALL_1]]:
+; CHECK-NEXT:    [[TMP2:%.*]] = call i32 @jt_target_1()
+; CHECK-NEXT:    br [[DOTTAIL]]
+; CHECK:       [[_TAIL:.*:]]
+; CHECK-NEXT:    [[TMP3:%.*]] = phi i32 [ [[TMP1]], %[[CALL_0]] ], [ [[TMP2]], %[[CALL_1]] ]
+; CHECK-NEXT:    ret i32 [[TMP3]]
+;
+  %gep = getelementptr inbounds [2 x ptr], ptr @jt, i32 0, i32 %idx
+  %fptr = load ptr, ptr %gep
+  %r = call i32 %fptr(), !prof !2
+  ret i32 %r
+}
+
 ;; VP metadata: GUID 11912887233601027218 = MD5("jt_target_0"), count 100
 ;;              GUID 18156790114353049777 = MD5("jt_target_1"), count 50
 !0 = !{!"VP", i32 0, i64 150, i64 11912887233601027218, i64 100, i64 18156790114353049777, i64 50}
 
 ;; VP metadata with multiple zero values.
 !1 = !{!"VP", i32 0, i64 150, i64 11912887233601027218, i64 60, i64 0, i64 50, i64 0, i64 40}
 
+;; VP metadata with only a zero value.
+!2 = !{!"VP", i32 0, i64 150, i64 0, i64 40}
+
 ;.
 ; CHECK: [[PROF0]] = !{!"branch_weights", i32 0, i32 100, i32 50}
 ; CHECK: [[PROF1]] = !{!"branch_weights", i32 0, i32 60, i32 0}
+; CHECK: [[PROF2]] = !{!"unknown", !"jump-table-to-switch"}
 ;.