[llvm-commits] [llvm] r45403 - in /llvm/trunk/lib/Transforms/Scalar: InstructionCombining.cpp SCCP.cpp

[Chris Lattner]

On Dec 28, 2007, at 11:56 PM, Christopher Lamb wrote:
> URL: http://llvm.org/viewvc/llvm-project?rev=45403&view=rev
> Log:
> Disable null pointer folding transforms for non-generic address  
> spaces. This should probably be a target-specific predicate based  
> on address space. That way for targets where this isn't applicable  
> the predicate can be optimized away.

Hi Christopher,

Is this really needed?  There are a variety of places in the compiler  
that assume that a null pointer load/store is invalid.  For example,  
I think we turn "x = load p;  c = p == null" into "x = load p; c =  
false" for example.  Is there any other solution to the problem  
you've hit?

-Chris

> Modified:
>     llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp
>     llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
>
> Modified: llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/ 
> Scalar/InstructionCombining.cpp?rev=45403&r1=45402&r2=45403&view=diff
>
> ====================================================================== 
> ========
> --- llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp  
> (original)
> +++ llvm/trunk/lib/Transforms/Scalar/InstructionCombining.cpp Sat  
> Dec 29 01:56:53 2007
> @@ -9338,8 +9338,11 @@
>          return ReplaceInstUsesWith(LI, LIB);
>    }
>
> -  if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op))
> -    if (isa<ConstantPointerNull>(GEPI->getOperand(0))) {
> +  if (GetElementPtrInst *GEPI = dyn_cast<GetElementPtrInst>(Op)) {
> +    const Value *GEPI0 = GEPI->getOperand(0);
> +    // TODO: Consider a target hook for valid address spaces for  
> this xform.
> +    if (isa<ConstantPointerNull>(GEPI0) &&
> +        cast<PointerType>(GEPI0->getType())->getAddressSpace() ==  
> 0) {
>        // Insert a new store to null instruction before the load to  
> indicate
>        // that this code is not reachable.  We do this instead of  
> inserting
>        // an unreachable instruction directly because we cannot  
> modify the
> @@ -9348,10 +9351,13 @@
>                      Constant::getNullValue(Op->getType()), &LI);
>        return ReplaceInstUsesWith(LI, UndefValue::get(LI.getType()));
>      }
> +  }
>
>    if (Constant *C = dyn_cast<Constant>(Op)) {
>      // load null/undef -> undef
> -    if ((C->isNullValue() || isa<UndefValue>(C))) {
> +    // TODO: Consider a target hook for valid address spaces for  
> this xform.
> +    if (isa<UndefValue>(C) || (C->isNullValue() &&
> +        cast<PointerType>(Op->getType())->getAddressSpace() == 0)) {
>        // Insert a new store to null instruction before the load to  
> indicate that
>        // this code is not reachable.  We do this instead of  
> inserting an
>        // unreachable instruction directly because we cannot modify  
> the CFG.
>
> Modified: llvm/trunk/lib/Transforms/Scalar/SCCP.cpp
> URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Transforms/ 
> Scalar/SCCP.cpp?rev=45403&r1=45402&r2=45403&view=diff
>
> ====================================================================== 
> ========
> --- llvm/trunk/lib/Transforms/Scalar/SCCP.cpp (original)
> +++ llvm/trunk/lib/Transforms/Scalar/SCCP.cpp Sat Dec 29 01:56:53 2007
> @@ -1015,7 +1015,9 @@
>    if (PtrVal.isUndefined()) return;   // The pointer is not  
> resolved yet!
>    if (PtrVal.isConstant() && !I.isVolatile()) {
>      Value *Ptr = PtrVal.getConstant();
> -    if (isa<ConstantPointerNull>(Ptr)) {
> +    // TODO: Consider a target hook for valid address spaces for  
> this xform.
> +    if (isa<ConstantPointerNull>(Ptr) &&
> +        cast<PointerType>(Ptr->getType())->getAddressSpace() == 0) {
>        // load null -> null
>        markConstant(IV, &I, Constant::getNullValue(I.getType()));
>        return;
>
>
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