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doh: fix undefined behaviour and open up for gcc and clang optimization

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The undefined behaviour is annoying when running fuzzing with
sanitizers. The codegen is the same, but the meaning is now not up for
dispute. See https://cppinsights.io/s/516a2ff4

By incrementing the pointer first, both gcc and clang recognize this as
a bswap and optimizes it to a single instruction.  See
https://godbolt.org/z/994Zpx

Closes #4350
This commit is contained in:
Paul Dreik 2019-09-13 20:06:21 +02:00 committed by Daniel Stenberg
parent b766602729
commit dda418266c
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1 changed files with 8 additions and 2 deletions
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10
lib/doh.c
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@ -432,8 +432,14 @@ static unsigned short get16bit(unsigned char *doh, int index)
static unsigned int get32bit(unsigned char *doh, int index)
{
return (doh[index] << 24) | (doh[index + 1] << 16) |
(doh[index + 2] << 8) | doh[index + 3];
/* make clang and gcc optimize this to bswap by incrementing
the pointer first. */
doh += index;
/* avoid undefined behaviour by casting to unsigned before shifting
24 bits, possibly into the sign bit. codegen is same, but
ub sanitizer won't be upset */
return ( (unsigned)doh[0] << 24) | (doh[1] << 16) |(doh[2] << 8) | doh[3];
}
static DOHcode store_a(unsigned char *doh, int index, struct dohentry *d)