[InstSimplify] Add trivial simplifications for gc.relocate intrinsic (#81639)

Fold gc.relocate of undef and null to undef and null respectively.

Similar transform is currently done by instcombine, but there is no
reason to not include it here as well.

Author: danilaml

llvm/lib/Analysis/InstructionSimplify.cpp

@@ -39,6 +39,7 @@
 #include "llvm/IR/Instructions.h"
 #include "llvm/IR/Operator.h"
 #include "llvm/IR/PatternMatch.h"
+#include "llvm/IR/Statepoint.h"
 #include "llvm/Support/KnownBits.h"
 #include <algorithm>
 #include <optional>
@@ -6847,6 +6848,27 @@ static Value *simplifyIntrinsic(CallBase *Call, Value *Callee,
   }
   case Intrinsic::experimental_constrained_ldexp:
     return simplifyLdexp(Args[0], Args[1], Q, true);
+  case Intrinsic::experimental_gc_relocate: {
+    GCRelocateInst &GCR = *cast<GCRelocateInst>(Call);
+    Value *DerivedPtr = GCR.getDerivedPtr();
+    Value *BasePtr = GCR.getBasePtr();
+
+    // Undef is undef, even after relocation.
+    if (isa<UndefValue>(DerivedPtr) || isa<UndefValue>(BasePtr)) {
+      return UndefValue::get(GCR.getType());
+    }
+
+    if (auto *PT = dyn_cast<PointerType>(GCR.getType())) {
+      // For now, the assumption is that the relocation of null will be null
+      // for most any collector. If this ever changes, a corresponding hook
+      // should be added to GCStrategy and this code should check it first.
+      if (isa<ConstantPointerNull>(DerivedPtr)) {
+        // Use null-pointer of gc_relocate's type to replace it.
+        return ConstantPointerNull::get(PT);
+      }
+    }
+    return nullptr;
+  }
   default:
     return nullptr;
   }

llvm/test/Transforms/InstSimplify/gc_relocate.ll

@@ -11,9 +11,34 @@ define void @dead_relocate(ptr addrspace(1) %in) gc "statepoint-example" {
 ;
 entry:
   %safepoint_token = call token (i64, i32, ptr, i32, i32, ...) @llvm.experimental.gc.statepoint.p0(i64 0, i32 0, ptr elementtype(void ()) @func, i32 0, i32 0, i32 0, i32 0) ["gc-live"(ptr addrspace(1) %in)]
-  %a = call ptr addrspace(1) @llvm.experimental.gc.relocate.p1(token %safepoint_token,  i32 0, i32 0)
+  %a = call ptr addrspace(1) @llvm.experimental.gc.relocate.p1(token %safepoint_token, i32 0, i32 0)
   ret void
 }
 
+define ptr addrspace(1) @relocate_undef() gc "statepoint-example" {
+; CHECK-LABEL: @relocate_undef(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SAFEPOINT_TOKEN:%.*]] = call token (i64, i32, ptr, i32, i32, ...) @llvm.experimental.gc.statepoint.p0(i64 0, i32 0, ptr elementtype(void ()) @func, i32 0, i32 0, i32 0, i32 0) [ "gc-live"(ptr addrspace(1) undef) ]
+; CHECK-NEXT:    ret ptr addrspace(1) undef
+;
+entry:
+  %safepoint_token = call token (i64, i32, ptr, i32, i32, ...) @llvm.experimental.gc.statepoint.p0(i64 0, i32 0, ptr elementtype(void ()) @func, i32 0, i32 0, i32 0, i32 0) ["gc-live"(ptr addrspace(1) undef)]
+  %a = call ptr addrspace(1) @llvm.experimental.gc.relocate.p1(token %safepoint_token, i32 0, i32 0)
+  ret ptr addrspace(1) %a
+}
+
+define ptr addrspace(1) @relocate_null() gc "statepoint-example" {
+; CHECK-LABEL: @relocate_null(
+; CHECK-NEXT:  entry:
+; CHECK-NEXT:    [[SAFEPOINT_TOKEN:%.*]] = call token (i64, i32, ptr, i32, i32, ...) @llvm.experimental.gc.statepoint.p0(i64 0, i32 0, ptr elementtype(void ()) @func, i32 0, i32 0, i32 0, i32 0) [ "gc-live"(ptr addrspace(1) null) ]
+; CHECK-NEXT:    ret ptr addrspace(1) null
+;
+entry:
+  %safepoint_token = call token (i64, i32, ptr, i32, i32, ...) @llvm.experimental.gc.statepoint.p0(i64 0, i32 0, ptr elementtype(void ()) @func, i32 0, i32 0, i32 0, i32 0) ["gc-live"(ptr addrspace(1) null)]
+  %a = call ptr addrspace(1) @llvm.experimental.gc.relocate.p1(token %safepoint_token, i32 0, i32 0)
+  ret ptr addrspace(1) %a
+}
+
+
 declare token @llvm.experimental.gc.statepoint.p0(i64, i32, ptr, i32, i32, ...)
 declare ptr addrspace(1) @llvm.experimental.gc.relocate.p1(token, i32, i32)