What to optimize:
-----------------
If you plan to do non-x86 architecture specific optimizations (SIMD normally),
-then take a look in the i386/ directory, as most important functions are
+then take a look in the x86/ directory, as most important functions are
already optimized for MMX.
If you want to do x86 optimizations then you can either try to finetune the
-stuff in the i386 directory or find some other functions in the C source to
+stuff in the x86 directory or find some other functions in the C source to
optimize, but there aren't many left.
As many functions tend to be a bit difficult to understand because
of optimizations, it can be hard to optimize them further, or write
architecture-specific versions. It is recommended to look at older
-revisions of the interesting files (for a web frontend try ViewVC at
-http://svn.mplayerhq.hu/ffmpeg/trunk/).
+revisions of the interesting files (web frontends for the various Libav
+branches are listed at http://libav.org/download.html).
Alternatively, look into the other architecture-specific versions in
-the i386/, ppc/, alpha/ subdirectories. Even if you don't exactly
+the x86/, ppc/, alpha/ subdirectories. Even if you don't exactly
comprehend the instructions, it could help understanding the functions
and how they can be optimized.
NOTE: If you still don't understand some function, ask at our mailing list!!!
-(http://lists.mplayerhq.hu/mailman/listinfo/ffmpeg-devel)
+(https://lists.libav.org/mailman/listinfo/libav-devel)
When is an optimization justified?
Some instructions on some architectures have strict alignment restrictions,
for example most SSE/SSE2 instructions on x86.
The minimum guaranteed alignment is written in the .h files, for example:
- void (*put_pixels_clamped)(const DCTELEM *block/*align 16*/, UINT8 *pixels/*align 8*/, int line_size);
+ void (*put_pixels_clamped)(const int16_t *block/*align 16*/, UINT8 *pixels/*align 8*/, int line_size);
General Tips:
__asm__(
"1: ....
...
- "jump_instruciton ....
+ "jump_instruction ....
Do not use C loops:
do{
__asm__(
...
}while()
-Use __asm__() instead of intrinsics. The latter requires a good optimizing compiler
-which gcc is not.
+For x86, mark registers that are clobbered in your asm. This means both
+general x86 registers (e.g. eax) as well as XMM registers. This last one is
+particularly important on Win64, where xmm6-15 are callee-save, and not
+restoring their contents leads to undefined results. In external asm (e.g.
+yasm), you do this by using:
+cglobal functon_name, num_args, num_regs, num_xmm_regs
+In inline asm, you specify clobbered registers at the end of your asm:
+__asm__(".." ::: "%eax").
+If gcc is not set to support sse (-msse) it will not accept xmm registers
+in the clobber list. For that we use two macros to declare the clobbers.
+XMM_CLOBBERS should be used when there are other clobbers, for example:
+__asm__(".." ::: XMM_CLOBBERS("xmm0",) "eax");
+and XMM_CLOBBERS_ONLY should be used when the only clobbers are xmm registers:
+__asm__(".." :: XMM_CLOBBERS_ONLY("xmm0"));
+
+Do not expect a compiler to maintain values in your registers between separate
+(inline) asm code blocks. It is not required to. For example, this is bad:
+__asm__("movdqa %0, %%xmm7" : src);
+/* do something */
+__asm__("movdqa %%xmm7, %1" : dst);
+- first of all, you're assuming that the compiler will not use xmm7 in
+ between the two asm blocks. It probably won't when you test it, but it's
+ a poor assumption that will break at some point for some --cpu compiler flag
+- secondly, you didn't mark xmm7 as clobbered. If you did, the compiler would
+ have restored the original value of xmm7 after the first asm block, thus
+ rendering the combination of the two blocks of code invalid
+Code that depends on data in registries being untouched, should be written as
+a single __asm__() statement. Ideally, a single function contains only one
+__asm__() block.
+
+Use external asm (nasm/yasm) or inline asm (__asm__()), do not use intrinsics.
+The latter requires a good optimizing compiler which gcc is not.
+
+Inline asm vs. external asm
+---------------------------
+Both inline asm (__asm__("..") in a .c file, handled by a compiler such as gcc)
+and external asm (.s or .asm files, handled by an assembler such as yasm/nasm)
+are accepted in Libav. Which one to use differs per specific case.
+
+- if your code is intended to be inlined in a C function, inline asm is always
+ better, because external asm cannot be inlined
+- if your code calls external functions, yasm is always better
+- if your code takes huge and complex structs as function arguments (e.g.
+ MpegEncContext; note that this is not ideal and is discouraged if there
+ are alternatives), then inline asm is always better, because predicting
+ member offsets in complex structs is almost impossible. It's safest to let
+ the compiler take care of that
+- in many cases, both can be used and it just depends on the preference of the
+ person writing the asm. For new asm, the choice is up to you. For existing
+ asm, you'll likely want to maintain whatever form it is currently in unless
+ there is a good reason to change it.
+- if, for some reason, you believe that a particular chunk of existing external
+ asm could be improved upon further if written in inline asm (or the other
+ way around), then please make the move from external asm <-> inline asm a
+ separate patch before your patches that actually improve the asm.
Links:
http://infocenter.arm.com/help/topic/com.arm.doc.ddi0211j/DDI0211J_arm1136_r1p5_trm.pdf
Optimization guide for Intel XScale (used in Sharp Zaurus PDA):
http://download.intel.com/design/intelxscale/27347302.pdf
-Intel Wireless MMX2 Coprocessor: Programmers Reference Manual
+Intel Wireless MMX 2 Coprocessor: Programmers Reference Manual
http://download.intel.com/design/intelxscale/31451001.pdf
PowerPC-specific:
projects / ffmpeg / blobdiff