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Clarify comments in ARMv6 divider regarding special-case handling of large (high bit set) numerators.

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git-svn-id: svn://svn.rockbox.org/rockbox/trunk@24783 a1c6a512-1295-4272-9138-f99709370657
This commit is contained in:
Andrew Mahone 2010-02-20 06:29:23 +00:00
parent 3716abba92
commit 950b2dfa2c
2 changed files with 9 additions and 5 deletions
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10
apps/codecs/demac/libdemac/udiv32_arm.S
View file

@ -234,10 +234,12 @@ udiv32_arm:
mul \inv, \divisor, \neg
smlawt \divisor, \divisor, \inv, \divisor
mul \inv, \divisor, \neg
/* This will save a cycle on ARMv6, but does not produce a correct result
if numerator sign bit is set. This case accounts for about 1 in 10^7 of
divisions, done by the APE decoder, so we specialize for the more common
case and handle the uncommon large-numerator separately */
/* This will save a cycle on ARMv6, but requires that the numerator sign
bit is not set (that of inv is guaranteed unset). The branch should
predict very well, making it typically 1 cycle, and thus both the branch
and test fill delay cycles for the multiplies. Based on logging of
numerator sizes in the APE codec, the branch is taken about 1/10^7 of
the time. */
#if ARM_ARCH >= 6
tst \numerator, \numerator
smmla \divisor, \divisor, \inv, \divisor

4
firmware/target/arm/support-arm.S
View file

@ -244,7 +244,9 @@
tst \numerator, \numerator
smmla \divisor, \divisor, \inv, \divisor
/* Branch to large-numerator handler, or else use smmul if sign bit is not
set. */
set. This wins on average with random numerators, and should be no
slower than using umull for small numerator, even if prediction fails.
*/
bmi 40f
smmul \inv, \numerator, \divisor
#else