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Re: Compiler optimizing variables in inline assembly
- From: Andrew Haley <aph at redhat dot com>
- To: Cody Rigney <codyrigney92 at gmail dot com>
- Cc: gcc-help at gcc dot gnu dot org
- Date: Fri, 21 Feb 2014 09:53:34 +0000
- Subject: Re: Compiler optimizing variables in inline assembly
- Authentication-results: sourceware.org; auth=none
- References: <CA+1=iYaWg6OyzNjM9K2Qb1fn40ei0Ls+3AhVyXcg-h2Pm3xQaw at mail dot gmail dot com> <5305C77D dot 3090807 at redhat dot com> <CA+1=iYbK=iDVKhvPESrbseF5nsB-U2oBBBjTiEyveuEkxVeZGw at mail dot gmail dot com>
On 02/20/2014 07:29 PM, Cody Rigney wrote:
> On Thu, Feb 20, 2014 at 4:14 AM, Andrew Haley <aph@redhat.com> 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.