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opus: inline EC_ILOG on ARMv4

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ARMv4 has no CLZ, so all 8,680 ilog2 calls went through libgcc's
__clzsi2.  Fifteen branchless instructions replace it.

Modelled: -0.91% ARMv4; ARMv5E unaffected, it already emits CLZ.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Change-Id: Ib84b97acb687da4101e02bad1b0e8bbd256d32df
This commit is contained in:
Michael Giacomelli 2026-09-18 11:59:22 -04:00 • committed by Solomon Peachy
parent d1fab121f8
commit 31f2a27f1e
1 changed files with 20 additions and 0 deletions
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20
lib/rbcodec/codecs/libopus/celt/ecintrin.h
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@ -64,6 +64,9 @@ static __inline int ec_bsr(unsigned long _x){
# include "dsplib.h"
# define EC_CLZ0 (31)
# define EC_CLZ(_x) (_lnorm(_x))
#elif defined(OPUS_ARM_INLINE_ASM) && !defined(OPUS_ARM_NO_ILOG_INLINE)
/*ARMv4 has no CLZ instruction, so __builtin_clz compiles to a call to
libgcc's __clzsi2. Leave EC_CLZ undefined and inline EC_ILOG below.*/
#elif __GNUC_PREREQ(3,4)
# if INT_MAX>=2147483647
# define EC_CLZ0 ((int)sizeof(unsigned)*CHAR_BIT)
@ -80,6 +83,23 @@ static __inline int ec_bsr(unsigned long _x){
The majority of the time we can never pass it zero.
When we need to, it can be special cased.*/
# define EC_ILOG(_x) (EC_CLZ0-EC_CLZ(_x))
#elif defined(OPUS_ARM_INLINE_ASM) && !defined(OPUS_ARM_NO_ILOG_INLINE)
/*Fifteen branchless instructions, no memory access. The libgcc call it
replaces costs about half again once the call, its table load and the
caller's register saves are counted. Compares against 0x10000, 0x100 and
0x10 because those are ARM immediates and the equivalent masks are not.
Returns 0 for 0, where the builtin form is undefined.*/
static __inline__ int ec_ilog_armv4(opus_uint32 _v){
int ret;
ret=0;
if(_v>=0x10000){_v>>=16;ret=16;}
if(_v>=0x100){_v>>=8;ret+=8;}
if(_v>=0x10){_v>>=4;ret+=4;}
if(_v>=0x4){_v>>=2;ret+=2;}
/*_v is now 0..3, whose ilog is 0, 1, 2, 2.*/
return ret+(int)(_v&~(_v>>1));
}
# define EC_ILOG(_x) (ec_ilog_armv4(_x))
#else
int ec_ilog(opus_uint32 _v);
# define EC_ILOG(_x) (ec_ilog(_x))