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Avoid signed left shift undefined behaviour in sext_hwi

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gcc/
	PR other/67042
	* hwint.h (sext_hwi): Switch to unsigned for the left shift, and
	conditionalize the whole on __GNUC__.  Add fallback code
	depending neither on undefined nor implementation-defined behaviour.

From-SVN: r227008
This commit is contained in:
Mikael Morin 2015-08-19 13:42:36 +00:00
parent dce573c43f
commit d63c864834
2 changed files with 25 additions and 2 deletions
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9
gcc/ChangeLog
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@ -1,9 +1,16 @@
2015-08-19 Mikael Morin <mikael@gcc.gnu.org>
PR other/67042
* hwint.h (sext_hwi): Switch to unsigned for the left shift, and
conditionalize the whole on __GNUC__. Add fallback code
depending neither on undefined nor implementation-defined behaviour.
2015-08-19 Jiong Wang <jiong.wang@arm.com>
* config/aarch64/aarch64.c (aarch64_load_symref_appropriately): Replace
whitespaces with tab.
2015-08-13 Florian Weimer <fweimer@redhat.com>
2015-08-19 Florian Weimer <fweimer@redhat.com>
* prj.adb (For_Every_Project_Imported_Context.Recursive_Check_Context):
Move Name_Id_Set instantiation to the Prj package, to avoid trampolines.

18
gcc/hwint.h
View File

@ -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. */