diff options
Diffstat (limited to 'gcc/hwint.h')
| -rw-r--r-- | gcc/hwint.h | 18 |
1 files changed, 17 insertions, 1 deletions
diff --git a/gcc/hwint.h b/gcc/hwint.h index 3793986cf6c..4acbf8e79ca 100644 --- a/gcc/hwint.h +++ b/gcc/hwint.h @@ -244,11 +244,27 @@ sext_hwi (HOST_WIDE_INT src, unsigned int prec) if (prec == HOST_BITS_PER_WIDE_INT) return src; else +#if defined (__GNUC__) { + /* Take the faster path if the implementation-defined bits it's relying + on are implemented the way we expect them to be. Namely, conversion + from unsigned to signed preserves bit pattern, and right shift of + a signed value propagates the sign bit. + We have to convert from signed to unsigned and back, because when left + shifting signed values, any overflow is undefined behaviour. */ gcc_checking_assert (prec < HOST_BITS_PER_WIDE_INT); int shift = HOST_BITS_PER_WIDE_INT - prec; - return (src << shift) >> shift; + return ((HOST_WIDE_INT) ((unsigned HOST_WIDE_INT) src << shift)) >> shift; } +#else + { + /* Fall back to the slower, well defined path otherwise. */ + gcc_checking_assert (prec < HOST_BITS_PER_WIDE_INT); + HOST_WIDE_INT sign_mask = HOST_WIDE_INT_1 << (prec - 1); + HOST_WIDE_INT value_mask = (HOST_WIDE_INT_1U << prec) - HOST_WIDE_INT_1U; + return (((src & value_mask) ^ sign_mask) - sign_mask); + } +#endif } /* Zero extend SRC starting from PREC. */ |