[PATCH] D136882: ConstantFold: Fold out compare of addrspacecasted null with global

[Matt Arsenault via Phabricator via llvm-commits]

arsenm created this revision.
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null in addrspace(0) is defined to not point to any valid object.
Casting the pointer to another address space cannot make it point
to a valid object.

      

This is consistent with alias analysis's interpretation of null, e.g.

      

; Function: ptr_compare: 2 pointers, 0 call sites
;   MayAlias:   i32* %ptr, i32 addrspace(3)* @lds
define void @ptr_compare(i32* %ptr) {

  %load = load i32, i32 addrspace(3)* @lds
   store i32 0, i32* %ptr
   ret void

}

      

; Function: null_compare: 2 pointers, 0 call sites
;   NoAlias:    i32 addrspace(3)* @lds, i32* null
define void @null_compare() {

  %load = load i32, i32 addrspace(3)* @lds
  store i32 0, i32* null
  ret void

}

      

Fixes issue 58617, for the basic case. More complex cases,
like an offset from a GEP should also fold out. I'm not sure
it's appropriate for ConstantFolding to handle the general case,
which should use getUnderlyingObject.


https://reviews.llvm.org/D136882

Files:
  llvm/lib/IR/ConstantFold.cpp
  llvm/test/Assembler/constant-fold-compare-null-to-global-issue58617.ll


Index: llvm/test/Assembler/constant-fold-compare-null-to-global-issue58617.ll
===================================================================
--- llvm/test/Assembler/constant-fold-compare-null-to-global-issue58617.ll
+++ llvm/test/Assembler/constant-fold-compare-null-to-global-issue58617.ll
@@ -7,7 +7,7 @@
 define i1 @cmp_addrspacecast_null() {
 ; CHECK-LABEL: @cmp_addrspacecast_null(
 ; CHECK-NEXT:  bb0:
-; CHECK-NEXT:    ret i1 icmp eq (ptr addrspace(3) addrspacecast (ptr null to ptr addrspace(3)), ptr addrspace(3) @lds)
+; CHECK-NEXT:    ret i1 false
 ;
 bb0:
   ret i1 icmp eq (ptr addrspace(3) addrspacecast (ptr null to ptr addrspace(3)), ptr addrspace(3) @lds)
@@ -15,7 +15,7 @@
 
 define i1 @cmp_addrspacecast_null_commute() {
 ; CHECK-LABEL: @cmp_addrspacecast_null_commute(
-; CHECK-NEXT:    ret i1 icmp eq (ptr addrspace(3) addrspacecast (ptr null to ptr addrspace(3)), ptr addrspace(3) @lds)
+; CHECK-NEXT:    ret i1 false
 ;
   ret i1 icmp eq (ptr addrspace(3) @lds, ptr addrspace(3) addrspacecast (ptr null to ptr addrspace(3)))
 }
Index: llvm/lib/IR/ConstantFold.cpp
===================================================================
--- llvm/lib/IR/ConstantFold.cpp
+++ llvm/lib/IR/ConstantFold.cpp
@@ -1582,13 +1582,21 @@
 static Constant *constantFoldCompareGlobalToNull(CmpInst::Predicate Predicate,
                                                  Constant *C1, Constant *C2) {
   const GlobalValue *GV = dyn_cast<GlobalValue>(C2);
-  if (!GV || !C1->isNullValue())
+  if (!GV)
     return nullptr;
 
   // Don't try to evaluate aliases.  External weak GV can be null.
-  if (!isa<GlobalAlias>(GV) && !GV->hasExternalWeakLinkage() &&
+  if (isa<GlobalAlias>(GV) || GV->hasExternalWeakLinkage())
+    return nullptr;
+
+  // Strip away addrspacecast to see if this is derived from null.
+  C1 = C1->stripPointerCasts();
+
+  // A null pointer in the default address space cannot point to any valid
+  // object, regardless of the address space.
+  if (isa<ConstantPointerNull>(C1) &&
       !NullPointerIsDefined(nullptr /* F */,
-                            GV->getType()->getAddressSpace())) {
+                            cast<PointerType>(C1->getType())->getAddressSpace())) {
     if (Predicate == ICmpInst::ICMP_EQ)
       return ConstantInt::getFalse(C1->getContext());
     else if (Predicate == ICmpInst::ICMP_NE)


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