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gzio: Optimize by removing division.

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This commit is contained in:
Vladimir Serbinenko 2015-02-26 22:10:09 +01:00
parent 67a6a9512a
commit 7ba066419a
1 changed files with 17 additions and 1 deletions
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18
grub-core/io/gzio.c
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@ -1161,6 +1161,19 @@ grub_gzio_open (grub_file_t io, const char *name __attribute__ ((unused)))
return file; return file;
} }
static grub_uint8_t
mod_31 (grub_uint16_t v)
{
/* At most 2 iterations for any number that
we can get here.
In any case faster than real division. */
while (v > 0x1f)
v = (v & 0x1f) + (v >> 5);
if (v == 0x1f)
return 0;
return v;
}
static int static int
test_zlib_header (grub_gzio_t gzio) test_zlib_header (grub_gzio_t gzio)
{ {
@ -1178,7 +1191,10 @@ test_zlib_header (grub_gzio_t gzio)
return 0; return 0;
} }
if ((cmf * 256U + flg) % 31U) /* Usually it would be: (cmf * 256 + flg) % 31 != 0. */
/* But 256 == 8 (31). */
/* By multiplying by 4 and using 32 == 1 (31). We get our formula. */
if (mod_31 (cmf + flg * 4) != 0)
{ {
grub_error (GRUB_ERR_BAD_COMPRESSED_DATA, N_("unsupported gzip format")); grub_error (GRUB_ERR_BAD_COMPRESSED_DATA, N_("unsupported gzip format"));
return 0; return 0;