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mask_of_range: simplify high comparison

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To test c <= high, instead of testing the sign of (high + 1) - c, negate the
sign of high - c (as we're doing for c - low). This is a little easier to
read and shaves 2 instructions off the arm thumb build with
arm-none-eabi-gcc 7.3.1.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
This commit is contained in:
Gilles Peskine 2021-07-30 12:57:22 +02:00
parent 831fd766f3
commit a2dc0cc9ea
1 changed files with 5 additions and 3 deletions
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8
library/base64.c
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@ -44,9 +44,11 @@
static unsigned char mask_of_range( unsigned char low, unsigned char high,
unsigned char c )
{
unsigned low_mask = ( c - low ) >> 8;
unsigned high_mask = ( c - high - 1 ) >> 8;
return( ~low_mask & high_mask & 0xff );
/* low_mask is: 0 if low <= c, 0x...ff if low > c */
unsigned low_mask = ( (unsigned) c - low ) >> 8;
/* high_mask is: 0 if c <= high, 0x...ff if high > c */
unsigned high_mask = ( (unsigned) high - c ) >> 8;
return( ~( low_mask | high_mask ) & 0xff );
}
/* Given a value in the range 0..63, return the corresponding Base64 digit.