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

[Danila Malyutin via llvm-commits]

https://github.com/danilaml created https://github.com/llvm/llvm-project/pull/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.

>From 7146b1f12f92ab93a813a91a74673d5df84764b7 Mon Sep 17 00:00:00 2001
From: Danila Malyutin <dmalyutin at azul.com>
Date: Tue, 13 Feb 2024 21:42:32 +0400
Subject: [PATCH] [InstSimplify] Add trivial simplifications for gc.relocate
 intrinsic

Fold gc.relocate of undef and null to undef and null respectively.
---
 llvm/lib/Analysis/InstructionSimplify.cpp     | 22 +++++++++++++++
 .../Transforms/InstSimplify/gc_relocate.ll    | 27 ++++++++++++++++++-
 2 files changed, 48 insertions(+), 1 deletion(-)

diff --git a/llvm/lib/Analysis/InstructionSimplify.cpp b/llvm/lib/Analysis/InstructionSimplify.cpp
index 51e258d69e9e2e..333b38f221cfc4 100644
--- a/llvm/lib/Analysis/InstructionSimplify.cpp
+++ b/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;
   }
diff --git a/llvm/test/Transforms/InstSimplify/gc_relocate.ll b/llvm/test/Transforms/InstSimplify/gc_relocate.ll
index 3f6de8b3845a23..894e5ed76584aa 100644
--- a/llvm/test/Transforms/InstSimplify/gc_relocate.ll
+++ b/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)