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Avoid undefined behaviour when byteswapping

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`a << b` is undefined when `a` is negative, and `a >> b` is
implementation-defined. The correct thing to do here is to cast to
unsigned, swap the bytes there and then swap back.

This also improves performance on some compilers: Clang is smart enough
to recognise that we're byteswapping here and reduce it to a single
`bswap` instruction on x86_64, but only for the unsigned versions.
This commit is contained in:
AbigailBuccaneer 2020-08-24 18:04:37 +01:00
parent 0f2757f000
commit 6995a2e1ba
1 changed files with 5 additions and 22 deletions
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27
libraries/classparser/src/javaendian.h
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@ -22,21 +22,6 @@ inline uint16_t bigswap(uint16_t x)
return x; return x;
} }
inline int64_t bigswap(int64_t x)
{
return x;
}
inline int32_t bigswap(int32_t x)
{
return x;
}
inline int16_t bigswap(int16_t x)
{
return x;
}
#else #else
inline uint64_t bigswap(uint64_t x) inline uint64_t bigswap(uint64_t x)
{ {
@ -55,22 +40,20 @@ inline uint16_t bigswap(uint16_t x)
return (x >> 8) | (x << 8); return (x >> 8) | (x << 8);
} }
#endif
inline int64_t bigswap(int64_t x) inline int64_t bigswap(int64_t x)
{ {
return (x >> 56) | ((x << 40) & 0x00FF000000000000) | ((x << 24) & 0x0000FF0000000000) | return static_cast<int64_t>(bigswap(static_cast<uint64_t>(x)));
((x << 8) & 0x000000FF00000000) | ((x >> 8) & 0x00000000FF000000) |
((x >> 24) & 0x0000000000FF0000) | ((x >> 40) & 0x000000000000FF00) | (x << 56);
} }
inline int32_t bigswap(int32_t x) inline int32_t bigswap(int32_t x)
{ {
return (x >> 24) | ((x << 8) & 0x00FF0000) | ((x >> 8) & 0x0000FF00) | (x << 24); return static_cast<int32_t>(bigswap(static_cast<uint32_t>(x)));
} }
inline int16_t bigswap(int16_t x) inline int16_t bigswap(int16_t x)
{ {
return (x >> 8) | (x << 8); return static_cast<int16_t>(bigswap(static_cast<uint16_t>(x)));
} }
#endif
} }