Date: Mon, 23 Mar 2015 10:50:02 +1100 (EST) From: Bruce Evans <brde@optusnet.com.au> To: Dimitry Andric <dim@freebsd.org> Cc: svn-src-head@freebsd.org, svn-src-all@freebsd.org, src-committers@freebsd.org Subject: Re: svn commit: r280357 - head/bin/expr Message-ID: <20150323102927.U1476@besplex.bde.org> In-Reply-To: <201503222240.t2MMeWJs061829@svn.freebsd.org> References: <201503222240.t2MMeWJs061829@svn.freebsd.org>
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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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