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>
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