[rs4gc] avoid insert base computation instructions for deopt uses

[reames]

Summary

If we have a value live over a call which is used for deopt at the call, we know that the value must be a base pointer. We can avoid potentially inserting IR to materialize a base for this value.

I'm not actually sure this is worth landing. I'm posting it mostly because it came up in another discussion thread, and I wanted to make it concrete. If we're actually seeing cases this is profitable, we're probably better off tackling the insertion algorithm.

[skatkov]

Don't you think that it is a significant compile-time improvement and it makes sense to land anyway?
findBasePointers does not look like a cheap operation...

[skatkov]

ok, I convinced myself this is a general good improvement. LGTM then,

llvm/lib/Transforms/Scalar/RewriteStatepointsForGC.cpp

 }
 
 /// Find the required based pointers (and adjust the live set) for the given
 /// parse point.
 static void findBasePointers(DominatorTree &DT, DefiningValueMapTy &DVCache,
                              CallBase *Call,
                              PartiallyConstructedSafepointRecord &result) {
   MapVector<Value *, Value *> PointerToBase;
-  findBasePointers(result.LiveSet, PointerToBase, &DT, DVCache);
+  StatepointLiveSetTy PotentiallyDerivedPointers = result.LiveSet;
+  // We assume that all pointers passed to deopt are base pointers; as an
+  // optimization, we can use this to avoid seperately materializing the base
+  // pointer graph.  This is only relevant since we're very conservative about
+  // generating new conflict nodes during base pointer insertion.  If we were
+  // smarter there, this would be irrelevant.
+  if (auto Opt = Call->getOperandBundle(LLVMContext::OB_deopt))
+    for (Value *V : Opt->Inputs) {
+      if (!PotentiallyDerivedPointers.count(V))
+        continue;
+      PotentiallyDerivedPointers.remove(V);
+      PointerToBase[V] = V;
+    }
+  findBasePointers(PotentiallyDerivedPointers, PointerToBase, &DT, DVCache);
 
   if (PrintBasePointers) {
     errs() << "Base Pairs (w/o Relocation):\n";
     for (auto &Pair : PointerToBase) {
       errs() << " derived ";
       Pair.first->printAsOperand(errs(), false);
       errs() << " base ";
       Pair.second->printAsOperand(errs(), false);

llvm/test/Transforms/RewriteStatepointsForGC/base-pointers.ll

; CHECK-NEXT: br i1
   %bdv = phi i64 addrspace(1)* [ %obj, %entry ], [ %obj2, %merge ]
   br i1 %cnd, label %merge, label %next
 
 next:                                             ; preds = %merge
   call void @foo() [ "deopt"(i32 0, i32 -1, i32 0, i32 0, i32 0) ]
   ret i64 addrspace(1)* %bdv
 }
 
+; We know from the deopt use that %bdv must be a base value, and as
+; result can avoid materializing the extra copy of the BDV phi node.
+; (Even without a general forward analysis)
+define i64 addrspace(1)* @test6(i1 %cnd, i64 addrspace(1)* %obj, i64 addrspace(1)* %obj2) gc "statepoint-example" {
+; CHECK-LABEL: @test6
+entry:
+  br label %merge
+
+merge:                                            ; preds = %merge, %entry
+; CHECK-LABEL: merge:
+; CHECK-NEXT: phi
+; CHECK-NEXT: br i1
+  %bdv = phi i64 addrspace(1)* [ %obj, %entry ], [ %obj2, %merge ]
+  br i1 %cnd, label %merge, label %next
+
+next:                                             ; preds = %merge
+  call void @foo() [ "deopt"(i64 addrspace(1)* %bdv) ]
+  ret i64 addrspace(1)* %bdv
+}
+
 declare void @foo()

llvm/test/Transforms/RewriteStatepointsForGC/scalar-base-vector.ll

entry:
   ret i32 addrspace(1)* %ptr
 }
 
 define void @test6() gc "statepoint-example" {
 ; CHECK-LABEL: @test6(
 ; CHECK-NEXT:  bb:
 ; CHECK-NEXT:    br label [[HEADER:%.*]]
 ; CHECK:       header:
-; CHECK-NEXT:    [[TMP_BASE:%.*]] = phi i8 addrspace(1)* [ [[TMP6_BASE:%.*]], [[LATCH:%.*]] ], [ null, [[BB:%.*]] ], !is_base_value !0
-; CHECK-NEXT:    [[TMP:%.*]] = phi i8 addrspace(1)* [ [[TMP6:%.*]], [[LATCH]] ], [ undef, [[BB]] ]
+; CHECK-NEXT:    [[TMP:%.*]] = phi i8 addrspace(1)* [ [[TMP6:%.*]], [[LATCH:%.*]] ], [ undef, [[BB:%.*]] ]
 ; CHECK-NEXT:    br label [[BB10:%.*]]
 ; CHECK:       bb10:
-; CHECK-NEXT:    [[STATEPOINT_TOKEN:%.*]] = call token (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 0, void ()* @spam, i32 0, i32 0, i32 0, i32 0) [ "deopt"(i8 addrspace(1)* [[TMP]]), "gc-live"(i8 addrspace(1)* [[TMP]], i8 addrspace(1)* [[TMP_BASE]]) ]
-; CHECK-NEXT:    [[TMP_RELOCATED:%.*]] = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token [[STATEPOINT_TOKEN]], i32 1, i32 0)
-; CHECK-NEXT:    [[TMP_BASE_RELOCATED:%.*]] = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token [[STATEPOINT_TOKEN]], i32 1, i32 1)
+; CHECK-NEXT:    [[STATEPOINT_TOKEN:%.*]] = call token (i64, i32, void ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_isVoidf(i64 2882400000, i32 0, void ()* @spam, i32 0, i32 0, i32 0, i32 0) [ "deopt"(i8 addrspace(1)* [[TMP]]), "gc-live"(i8 addrspace(1)* [[TMP]]) ]
+; CHECK-NEXT:    [[TMP_RELOCATED:%.*]] = call coldcc i8 addrspace(1)* @llvm.experimental.gc.relocate.p1i8(token [[STATEPOINT_TOKEN]], i32 0, i32 0)
 ; CHECK-NEXT:    br label [[BB25:%.*]]
 ; CHECK:       bb25:
 ; CHECK-NEXT:    [[STATEPOINT_TOKEN1:%.*]] = call token (i64, i32, <2 x i8 addrspace(1)*> ()*, i32, i32, ...) @llvm.experimental.gc.statepoint.p0f_v2p1i8f(i64 2882400000, i32 0, <2 x i8 addrspace(1)*> ()* @baz, i32 0, i32 0, i32 0, i32 0)
 ; CHECK-NEXT:    [[TMP262:%.*]] = call <2 x i8 addrspace(1)*> @llvm.experimental.gc.result.v2p1i8(token [[STATEPOINT_TOKEN1]])
-; CHECK-NEXT:    [[BASE_EE:%.*]] = extractelement <2 x i8 addrspace(1)*> [[TMP262]], i32 0, !is_base_value !0
 ; CHECK-NEXT:    [[TMP27:%.*]] = extractelement <2 x i8 addrspace(1)*> [[TMP262]], i32 0
 ; CHECK-NEXT:    br i1 undef, label [[BB7:%.*]], label [[LATCH]]
 ; CHECK:       bb7:
 ; CHECK-NEXT:    br label [[LATCH]]
 ; CHECK:       latch:
-; CHECK-NEXT:    [[TMP6_BASE]] = phi i8 addrspace(1)* [ [[BASE_EE]], [[BB25]] ], [ [[BASE_EE]], [[BB7]] ], !is_base_value !0
 ; CHECK-NEXT:    [[TMP6]] = phi i8 addrspace(1)* [ [[TMP27]], [[BB25]] ], [ [[TMP27]], [[BB7]] ]
 ; CHECK-NEXT:    br label [[HEADER]]
 ;
 bb:
   br label %header
 
 header:                                              ; preds = %latch, %bb
   %tmp = phi i8 addrspace(1)* [ %tmp6, %latch ], [ undef, %bb ]