svn commit: r280357 - head/bin/expr
Bruce Evans
brde at optusnet.com.au
Mon Mar 23 00:07:11 UTC 2015
On Sun, 22 Mar 2015, Dimitry Andric wrote:
> Log:
> Build expr with -fwrapv, since it relies on signed integer wrapping
> having defined behavior.
This was supposed to be fixed using a volatile hack, but I see that
was only for multiplication.
Wrapping is not undefined behaviour. You mean overflow.
expr mainly relies on the undefined behaviour giving a result and not
causing the tests that the result is correct being optimized away. All
compilers on all normal systems give a result. The volatile hack
prevents the excessive optimization. But expr uses sloppy tests for
addition and subtraction. For multiplication and division, there is
no simple correctness test and expr does a complete test. For addition
and multiplication, it assumes that the overflow gives (2's complement)
wrapping so that simple sign tests work.
> Modified: head/bin/expr/Makefile
> ==============================================================================
> --- head/bin/expr/Makefile Sun Mar 22 22:00:29 2015 (r280356)
> +++ head/bin/expr/Makefile Sun Mar 22 22:40:32 2015 (r280357)
> @@ -6,6 +6,9 @@ PROG= expr
> SRCS= expr.y
> YFLAGS=
>
> +# expr relies on signed integer wrapping
> +CFLAGS+= -fwrapv
Hard-coded gcc flags make compiling with non-gcc compilers impossible.
-fwrapv implies 2's complement, so might not be supported even by gcc.
Bruce
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