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toolchain/39205: gcc-4.1.2 shifting a boolean may cause bad sign extension
>Synopsis: gcc-4.1.2 shifting a boolean may cause bad sign extension
>Arrival-Date: Fri Jul 25 14:15:00 +0000 2008
>Originator: Michael van Elst
>Release: NetBSD 4.0_STABLE
Michael van Elst
"A potential Snark may lurk in every tree."
System: NetBSD henery 4.0_STABLE NetBSD 4.0_STABLE (HENERY) #0: Tue Jul 22
20:55:01 CEST 2008
Shifting a boolean value may cause a bad sign extension with
gcc-4.1.2 as found in netbsd-4.
The following code
signed char x = -1;
y = (x < 0) << 7;
prints -128, the correct value is 1<<7 == 128.
Apparently the type of x is used as the type of the value being
shifted instead of 'int' which is the type of the comparison.
If you use a signed short then a shift of 15 causes the bad
However, (x < 0) << 8 seems to work fine again as well as
using a temporary variable for the boolean. I.e.
d = (x < 0);
y = d << 7;
produces the correct result, independent of d being of type
int or signed char.
Verified on i386/amd64/m68k.
gcc-4.1.3 (in -current) and gcc3 from pkgsrc generate good code.
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