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codecs: link libarm_support for its division routines

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lib/arm_support/support-arm.S was written to replace libgcc's
division for ARM, and bff5a35c3c (FS#10943, 2010) added it to the
core, the plugin library and the codec library alike.  When
1501df045f (2013) replaced EXTRA_LIBS with explicit lists, plugins
kept it and codecs did not, and they have taken their division from
libgcc since.

With the gcc 4.4 toolchain that cost little: its libgcc had a
routine that used clz.  With gcc 9.5 libgcc has no soft-float ARMv5
variant, so ARMv5 targets get the ARMv4 routine, a shift and
subtract loop of about 130 cycles a division.

Monkey's Audio divides two or three times a sample in its range
decoder and, without codec IRAM, does it in C.  On a Clip+ it is
11% to 21% slower than 3.14 was, with over half of -c1000's decode
in __udivsi3.  MP2 is 4% to 7% slower.

Put libarm_support back, ahead of libgcc.  In perfsim's model of
the Clip+ a division falls to 44 cycles, Monkey's Audio by 38%, 30%
and 22% at -c1000, -c2000 and -c3000 (60.9 to 37.9 MHz at -c1000),
and MP2 by 3% to 7%; nothing else moves by more than 0.6%.  A Clip+
decodes -c1000 with the same checksum as before.

A division by zero in a codec goes to __div0 again, and so to the
firmware's handler, as it did with support-arm.S and with the old
libgcc (3.14's ape.codec calls it).  The gcc 9.5 libgcc returns
from its own stub instead.

Change-Id: I6a9a79ce8e85bca69870349a5c0823f392a578b6
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
This commit is contained in:
Michael Giacomelli 2026-10-04 23:46:57 -04:00 • committed by Solomon Peachy
parent 4c78e98461
commit 5d8ce62b45
1 changed files with 3 additions and 2 deletions
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5
lib/rbcodec/codecs/codecs.make
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@ -17,8 +17,9 @@ CODECS := $(subst $(RBCODECLIB_DIR),$(RBCODEC_BLD),$(CODECS))
include $(RBCODECLIB_DIR)/codecs/lib/libcodec.make
OTHER_INC += -I$(RBCODECLIB_DIR)/codecs/lib
# extra libraries
CODEC_LIBS := $(CODECLIB) $(FIXEDPOINTLIB)
# extra libraries. libarm_support has the division routines, which are much
# faster than what libgcc may supply; it is empty when not building for ARM.
CODEC_LIBS := $(CODECLIB) $(FIXEDPOINTLIB) $(ARMSUPPORTLIB)
# compile flags for codecs
CODECFLAGS := $(CFLAGS) $(RBCODEC_CFLAGS) -fstrict-aliasing \