[FastISel] Fix crash for gc.relocate lowring

[skatkov]

Summary

Lowering safepoint checks that all gc.relocaes observed in safepoint
must be lowered. However Fast-Isel is able to skip dead gc.relocate.

To resolve this issue we just ignore dead gc.relocate in the check.

[reames]

Hm, I think it's better to weaken the invariant in this case. If the instruction is dead, then selectiondag shouldn't be lowering it or expecting to lower it.

[reames]

LGTM w/requested change applied in both places.

lib/CodeGen/SelectionDAG/StatepointLowering.h

70

idiomatic way to write this would be:
if (!RelocCall->use_empty())

lib/CodeGen/SelectionDAG/StatepointLowering.h

assert(!Locations.count(Val) &&
            "Trying to allocate already allocated location");
     Locations[Val] = Location;
   }
 
   /// Record the fact that we expect to encounter a given gc_relocate
   /// before the next statepoint.  If we don't see it, we'll report
   /// an assertion.
   void scheduleRelocCall(const CallInst &RelocCall) {
+    // We are not interested in lowering dead instructions.
+    if (RelocCall.hasNUsesOrMore(1))
       PendingGCRelocateCalls.push_back(&RelocCall);
   }
 
   /// Remove this gc_relocate from the list we're expecting to see
   /// before the next statepoint.  If we weren't expecting to see
   /// it, we'll report an assertion.
   void relocCallVisited(const CallInst &RelocCall) {
+    // We are not interested in lowering dead instructions.
+    if (!RelocCall.hasNUsesOrMore(1))
+      return;
     auto I = llvm::find(PendingGCRelocateCalls, &RelocCall);
     assert(I != PendingGCRelocateCalls.end() &&
            "Visited unexpected gcrelocate call");
     PendingGCRelocateCalls.erase(I);
   }
 
   // TODO: Should add consistency tracking to ensure we encounter
   // expected gc_result calls too.

test/CodeGen/X86/fast-isel-gc-intrinsics.ll

 
 ; we are okay here because we see the gcrelocate and avoid generating their own
 ; block.
 define i1 @test_gcresult_gcrelocate(i8 addrspace(1)* %v) gc "statepoint-example" {
 entry:
   %safepoint_token = tail call token (i64, i32, i1 ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_i1f(i64 0, i32 0, i1 ()* @return_i1, i32 0, i32 0, i32 0, i32 0, i8 addrspace(1)* %v)
   %call1 = call zeroext i1 @llvm.experimental.gc.result.i1(token %safepoint_token)
   %vnew = call i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token %safepoint_token,  i32 7, i32 7)
+  br label %exit
+exit:
   ret i1 %call1
 }
 
 define i8 addrspace(1)*  @test_non_entry_block(i8 addrspace(1)* %v, i8 %val) gc "statepoint-example" {
 entry:
  %load = load i8, i8 addrspace(1)* %v
  %cmp = icmp eq i8 %load, %val
  br i1 %cmp, label %func_call, label %exit