Loading from null is valid outside of addrspace 0

This patches fixes a miscompile where we were assuming that loading from null is undefined and thus we could assume it doesn't happen.  This transform is perfectly legal in address space 0, but is not neccessarily legal in other address spaces.

We really should introduce a hook to control this property on a per target per address space basis.  We may be loosing valuable optimizations in some address spaces by being too conservative.

Original patch by Thomas P Raoux (submitted to llvm-commits), tests and formatting fixes by me.

llvm-svn: 224961

2 files changed, 30 insertions, 10 deletions

diff --git a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
index b24f0eafd5d..0f796fee56a 100644
--- a/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
+++ b/llvm/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
@@ -474,18 +474,18 @@ Instruction *InstCombiner::visitLoadInst(LoadInst &LI) {
       }
 
       // load (select (cond, null, P)) -> load P
-      if (Constant *C = dyn_cast<Constant>(SI->getOperand(1)))
-        if (C->isNullValue()) {
-          LI.setOperand(0, SI->getOperand(2));
-          return &LI;
-        }
+      if (isa<ConstantPointerNull>(SI->getOperand(1)) && 
+          LI.getPointerAddressSpace() == 0) {
+        LI.setOperand(0, SI->getOperand(2));
+        return &LI;
+      }
 
       // load (select (cond, P, null)) -> load P
-      if (Constant *C = dyn_cast<Constant>(SI->getOperand(2)))
-        if (C->isNullValue()) {
-          LI.setOperand(0, SI->getOperand(1));
-          return &LI;
-        }
+      if (isa<ConstantPointerNull>(SI->getOperand(2)) &&
+          LI.getPointerAddressSpace() == 0) {
+        LI.setOperand(0, SI->getOperand(1));
+        return &LI;
+      }
     }
   }
   return nullptr;
diff --git a/llvm/test/Transforms/InstCombine/select.ll b/llvm/test/Transforms/InstCombine/select.ll
index 765b42dfbe2..054f00a0de4 100644
--- a/llvm/test/Transforms/InstCombine/select.ll
+++ b/llvm/test/Transforms/InstCombine/select.ll
@@ -308,6 +308,26 @@ define i32 @test16(i1 %C, i32* %P) {
 ; CHECK: ret i32 %V
 }
 
+;; It may be legal to load from a null address in a non-zero address space
+define i32 @test16_neg(i1 %C, i32 addrspace(1)* %P) {
+        %P2 = select i1 %C, i32 addrspace(1)* %P, i32 addrspace(1)* null
+        %V = load i32 addrspace(1)* %P2
+        ret i32 %V
+; CHECK-LABEL: @test16_neg
+; CHECK-NEXT: %P2 = select i1 %C, i32 addrspace(1)* %P, i32 addrspace(1)* null
+; CHECK-NEXT: %V = load i32 addrspace(1)* %P2
+; CHECK: ret i32 %V
+}
+define i32 @test16_neg2(i1 %C, i32 addrspace(1)* %P) {
+        %P2 = select i1 %C, i32 addrspace(1)* null, i32 addrspace(1)* %P
+        %V = load i32 addrspace(1)* %P2
+        ret i32 %V
+; CHECK-LABEL: @test16_neg2
+; CHECK-NEXT: %P2 = select i1 %C, i32 addrspace(1)* null, i32 addrspace(1)* %P
+; CHECK-NEXT: %V = load i32 addrspace(1)* %P2
+; CHECK: ret i32 %V
+}
+
 define i1 @test17(i32* %X, i1 %C) {
         %R = select i1 %C, i32* %X, i32* null           
         %RV = icmp eq i32* %R, null