Add a rather pointless InstructionSimplify transform, inspired by recent constant

folding improvements: if P points to a type of size zero, turn "gep P, N" into "P".
More generally, if a gep index type has size zero, instcombine could replace the
index with zero, but that is not done here.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119942 91177308-0d34-0410-b5e6-96231b3b80d8

lib/Analysis/InstructionSimplify.cpp

@@ -18,6 +18,7 @@
 #include "llvm/Analysis/Dominators.h"
 #include "llvm/Support/PatternMatch.h"
 #include "llvm/Support/ValueHandle.h"
+#include "llvm/Target/TargetData.h"
 using namespace llvm;
 using namespace llvm::PatternMatch;
 
@@ -698,11 +699,18 @@ Value *llvm::SimplifyGEPInst(Value *const *Ops, unsigned NumOps,
   //if (isa<UndefValue>(Ops[0]))
   //  return UndefValue::get(GEP.getType());
 
+  if (NumOps == 2) {
     // getelementptr P, 0 -> P.
-  if (NumOps == 2)
     if (ConstantInt *C = dyn_cast<ConstantInt>(Ops[1]))
       if (C->isZero())
         return Ops[0];
+    // getelementptr P, N -> P if P points to a type of zero size.
+    if (TD) {
+      const Type *Ty = cast<PointerType>(Ops[0]->getType())->getElementType();
+      if (Ty->isSized() && !TD->getTypeAllocSize(Ty))
+        return Ops[0];
+    }
+  }
 
   // Check to see if this is constant foldable.
   for (unsigned i = 0; i != NumOps; ++i)

test/Transforms/InstCombine/2010-11-21-SizeZeroTypeGEP.ll

@@ -0,0 +1,8 @@
+; RUN: opt < %s -instcombine -S | not grep getelementptr
+
+target datalayout = "e-p:64:64:64-i1:8:8-i8:8:8-i16:16:16-i32:32:32-i64:64:64-f32:32:32-f64:64:64-v64:64:64-v128:128:128-a0:0:64-s0:64:64-f80:128:128"
+
+define {}* @foo({}* %x, i32 %n) {
+  %p = getelementptr {}* %x, i32 %n
+  ret {}* %p
+}