[Mesa-dev] [PATCH] i965/tiled_memcpy: realign rgba8_copy_aligned_dst stack in 32-bit builds

[Ian Romanick]

On 03/20/2018 01:50 PM, Chris Wilson wrote:
> Quoting Scott D Phillips (2018-03-20 20:39:25)
>> When building intel_tiled_memcpy for i686, the stack will only be
>> 4-byte aligned. This isn't sufficient for SSE temporaries which
>> require 16-byte alignment.  Use the force_align_arg_pointer
>> function attribute in that case to ensure sufficient alignment.
>> ---
>>  src/mesa/drivers/dri/i965/intel_tiled_memcpy.c | 8 +++++++-
>>  1 file changed, 7 insertions(+), 1 deletion(-)
>>
>> diff --git a/src/mesa/drivers/dri/i965/intel_tiled_memcpy.c b/src/mesa/drivers/dri/i965/intel_tiled_memcpy.c
>> index 69306828d72..bd8bafbd2d7 100644
>> --- a/src/mesa/drivers/dri/i965/intel_tiled_memcpy.c
>> +++ b/src/mesa/drivers/dri/i965/intel_tiled_memcpy.c
>> @@ -42,6 +42,12 @@
>>  #include <emmintrin.h>
>>  #endif
>>  
>> +#if defined(__GNUC__) && defined(__i386__) && (defined(__SSSE3__) || defined(__SSE2__))
>> +#define REALIGN __attribute__((force_align_arg_pointer))
>> +#else
>> +#define REALIGN
>> +#endif
> 
> It would be a harmless no-op on x86-64 (or essential ;)
> 
>> +
>>  #define FILE_DEBUG_FLAG DEBUG_TEXTURE
>>  
>>  #define ALIGN_DOWN(a, b) ROUND_DOWN_TO(a, b)
>> @@ -156,7 +162,7 @@ rgba8_copy_16_aligned_src(void *dst, const void *src)
>>  /**
>>   * Copy RGBA to BGRA - swap R and B, with the destination 16-byte aligned.
>>   */
>> -static inline void *
>> +static REALIGN inline void *
>>  rgba8_copy_aligned_dst(void *dst, const void *src, size_t bytes)
>>  {
>>     assert(bytes == 0 || !(((uintptr_t)dst) & 0xf));
> 
> Hmm, if aligned_dst is spilling to stack, so would be aligned_src. As
> these are supposed to be inlined (and constant folded), do you not want
> to realign the callers instead? Perhaps with an explicit FLATTEN.

The particular bug seems to mostly only trigger in unoptimized debug
builds.  I couldn't get it to trigger with -O2, but with -O0 it was 100%
reproducible.

> -Chris
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