[llvm] r224961 - Loading from null is valid outside of addrspace 0

[Philip Reames]

Author: reames
Date: Mon Dec 29 16:46:21 2014
New Revision: 224961

URL: http://llvm.org/viewvc/llvm-project?rev=224961&view=rev
Log:
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.



Modified:
    llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
    llvm/trunk/test/Transforms/InstCombine/select.ll

Modified: llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp?rev=224961&r1=224960&r2=224961&view=diff
==============================================================================
--- llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp (original)
+++ llvm/trunk/lib/Transforms/InstCombine/InstCombineLoadStoreAlloca.cpp Mon Dec 29 16:46:21 2014
@@ -474,18 +474,18 @@ Instruction *InstCombiner::visitLoadInst
       }
 
       // 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;

Modified: llvm/trunk/test/Transforms/InstCombine/select.ll
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/test/Transforms/InstCombine/select.ll?rev=224961&r1=224960&r2=224961&view=diff
==============================================================================
--- llvm/trunk/test/Transforms/InstCombine/select.ll (original)
+++ llvm/trunk/test/Transforms/InstCombine/select.ll Mon Dec 29 16:46:21 2014
@@ -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