[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 ↗

(On Diff #193671)

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

lib/CodeGen/SelectionDAG/SelectionDAGISel.cpp

 /// side-effect free and is either dead or folded into a generated instruction.
 /// Return false if it needs to be emitted.
 static bool isFoldedOrDeadInstruction(const Instruction *I,
                                       FunctionLoweringInfo *FuncInfo) {
   return !I->mayWriteToMemory() && // Side-effecting instructions aren't folded.
          !I->isTerminator() &&     // Terminators aren't folded.
          !isa<DbgInfoIntrinsic>(I) &&  // Debug instructions aren't folded.
          !I->isEHPad() &&              // EH pad instructions aren't folded.
-         !FuncInfo->isExportedInst(I); // Exported instrs must be computed.
+         !FuncInfo->isExportedInst(I) && // Exported instrs must be computed.
+         !isGCRelocate(I); // GCRelocate cannot be skipped.
 }
 
 /// Set up SwiftErrorVals by going through the function. If the function has
 /// swifterror argument, it will be the first entry.
 static void setupSwiftErrorVals(const Function &Fn, const TargetLowering *TLI,
                                 FunctionLoweringInfo *FuncInfo) {
   if (!TLI->supportSwiftError())
     return;

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