diff options
2 files changed, 9 insertions, 5 deletions
diff --git a/apps/codecs/demac/libdemac/udiv32_arm.S b/apps/codecs/demac/libdemac/udiv32_arm.S
index 939fce17d4..10c0731db1 100644
--- a/apps/codecs/demac/libdemac/udiv32_arm.S
+++ b/apps/codecs/demac/libdemac/udiv32_arm.S
@@ -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
diff --git a/firmware/target/arm/support-arm.S b/firmware/target/arm/support-arm.S
index dd17e0f5f7..b4577f6e9b 100644
--- a/firmware/target/arm/support-arm.S
+++ b/firmware/target/arm/support-arm.S
@@ -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