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

[via llvm-commits]

llvmbot wrote:


<!--LLVM PR SUMMARY COMMENT-->

@llvm/pr-subscribers-llvm-analysis

Author: Danila Malyutin (danilaml)

<details>
<summary>Changes</summary>

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.

---
Full diff: https://github.com/llvm/llvm-project/pull/81639.diff


2 Files Affected:

- (modified) llvm/lib/Analysis/InstructionSimplify.cpp (+22) 
- (modified) llvm/test/Transforms/InstSimplify/gc_relocate.ll (+26-1) 


``````````diff
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)

``````````

</details>


https://github.com/llvm/llvm-project/pull/81639