Back in June I briefly mentioned in one of my gcc-patches posts that
a change that should have always reduced code size, would mysteriously
occasionally result in slightly larger code (according to CSiBE):
https://gcc.gnu.org/pipermail/gcc-patches/2021-June/573233.html
Investigating further, the cause turns out to be that x86_64's
scalar-to-vector (stv) pass is relying on poor estimates of the size
costs/benefits. This patch tweaks the backend's compute_convert_gain
method to provide slightly more accurate values when compiling with
-Os. Compilation without -Os is (should be) unaffected. And for
completeness, I'll mention that the stv pass is a net win for code
size so it's much better to improve its heuristics than simply gate
the pass on !optimize_for_size.
The net effect of this change is to save 1399 bytes on the CSiBE
code size benchmark when compiling with -Os.
This patch has been tested on x86_64-pc-linux-gnu with "make bootstrap"
and "make -k check" with no new failures.
Ok for mainline?
2021-08-19 Roger Sayle <ro...@nextmovesoftware.com>
gcc/ChangeLog
* config/i386/i386-features.c (compute_convert_gain): Provide
more accurate values for CONST_INT, when optimizing for size.
* config/i386/i386.c (COSTS_N_BYTES): Move definition from here...
* config/i386/i386.h (COSTS_N_BYTES): to here.
Roger
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