[llvm-commits] [llvm] r51134 - in /llvm/trunk: lib/Transforms/Scalar/InstructionCombining.cpp test/Transforms/InstCombine/2003-11-13-ConstExprCastCall.ll test/Transforms/InstCombine/fp-ret-bitcast.ll

[Duncan Sands]

Hi Bill,

> Situations can arise when you have a function called that returns a 'void', but
> is bitcast to return a floating point value. The result of the instruction may
> not be used by the program afterwards, and LLVM will happily remove all
> instructions except the call. But, on some platforms, if a value is returned as
> a floating point, it may need to be removed from the stack (like x87). Thus, we
> can't get rid of the bitcast even if there isn't a use of the value.

I don't get it.  Running the testcase under instcombine without your changes
turns

%tmp107 = call float bitcast (void (%struct.NSObject*, ...)* @objc_msgSend_fpret to float (%struct.NSObject*, %struct.objc_selector*)*)( %struct.NSObject* %tmp106, %struct.objc_selector* %tmp105 ) nounwind              ; <float> [#uses=0]

into

        call void (%struct.NSObject*, ...)* @objc_msgSend_fpret( %struct.NSObject* %tmp106, %struct.objc_selector* %tmp105 ) nounwind

Since there is now no floating point value returned, how can there be a problem with
removing it from the stack?  So I don't understand what this change is fixing.

Ciao,

Duncan.