I added all the registers and "memory" to clobber and it worked! Since I think it's more efficient to specify which memory is changing, I'm going to give the "=m" (*a) a shot. So even though you dereference the pointer, gcc will know the length of the memory(say 2 bytes) that changes? Or do you have to specify each index of the memory(e.g. char a[2]; ..... asm ... "=m" (*a), "=m" (*(a+1)))? On Fri, Feb 21, 2014 at 4:53 AM, Andrew Haley <aph@xxxxxxxxxx> wrote: > On 02/20/2014 07:29 PM, Cody Rigney wrote: >> On Thu, Feb 20, 2014 at 4:14 AM, Andrew Haley <aph@xxxxxxxxxx> wrote: >>> Hi, >>> >>> On 02/19/2014 07:04 PM, Cody Rigney wrote: >>>> I'm trying to add NEON optimizations to OpenCV's LK optical flow. See >>>> link below. >>>> https://github.com/Itseez/opencv/blob/2.4/modules/video/src/lkpyramid.cpp >>>> >>>> The gcc version could vary since this is an open source project, but >>>> the one I'm currently using is 4.8.1. The target architecture is ARMv7 >>>> w/ NEON. The processor I'm testing on is an ARM >>>> Cortex-A15(big.LITTLE). >>>> >>>> The problem is, in release mode (where optimizations are set) it does >>>> not work properly. However, in debug mode, it works fine. I tracked >>>> down a specific variable(FLT_SCALE) that was being optimized out and >>>> made it volatile and that part worked fine after that. However, I'm >>>> still having incorrect behavior from some other optimization. >>> >>> Forget about using volatile here. That's just wrong. >>> >>> You have to mark your inputs, outputs, and clobbers correctly. >>> >> That makes sense. In this case, the input parameters are actually >> memory addresses. So how would I do an output or clobber that would >> tell the compiler that the memory at those addresses will change? > > You can use a memory operand as an output, as in "=m"(*a) or simply > add "memory" to the clobber list. And you must add all clobbered > registers to the clobber list. Then it should work. > > Andrew. > >
