Re: -fstrict-overflow and -Wstrict-overflow

[SAITOH Masanobu <msaitoh%execsw.org@localhost>]

(2012/10/13 3:13), Alan Barrett wrote:
> On Fri, 12 Oct 2012, Masanobu SAITOH wrote:
>> One of (serious) problem that we can't use -Wstrict-overflow by default
>> is that gcc sometimes misundestand it :-<
>>
>> How should we use it?
> 
> Do you have an example where gcc does the wrong thing with -Wstrict-overflow? 
>  (The example in your message, which I am not quoting here, showed gcc doing 
> the right thing: that is, drawing attention to a case where the program 
> invoked undefined behaviour.)

 One of example is src/lib/libm/src/e_jn.c

> five(msaitoh)% env USETOOLS=no make e_jn.o
> #   compile  libm/e_jn.o
> gcc -O2 -Wstrict-overflow -std=gnu99  -Wall -Wstrict-prototypes 
> -Wmissing-prototypes -Wpointer-arith -Wno-sign-compare  -Wno-traditional  
> -Wa,--fatal-warnings -Wreturn-type -Wswitch -Wshadow -Wcast-qual 
> -Wwrite-strings -Wextra -Wno-unused-parameter -Wno-sign-compare 
> -Wold-style-definition -Wsign-compare -Wformat=2   -Wno-format-zero-length  
> -Werror   -fno-strict-aliasing  -D_MULTI_LIBM -D_POSIX_MODE -DLIBM_SCCS 
> -I/disk/sources/4NetBSD-current/src/lib/libm/gen  -c    
> /disk/sources/4NetBSD-current/src/lib/libm/src/e_jn.c -o e_jn.o
> cc1: warnings being treated as errors
> /disk/sources/4NetBSD-current/src/lib/libm/src/e_jn.c: In function 
> '__ieee754_jn':
> /disk/sources/4NetBSD-current/src/lib/libm/src/e_jn.c:75:4: error: assuming 
> signed overflow does not occur when simplifying conditional to constant
> /disk/sources/4NetBSD-current/src/lib/libm/src/e_jn.c:107:10: error: assuming 
> signed overflow does not occur when simplifying conditional to constant
> /disk/sources/4NetBSD-current/src/lib/libm/src/e_jn.c:184:11: error: assuming 
> signed overflow does not occur when simplifying conditional to constant
> /disk/sources/4NetBSD-current/src/lib/libm/src/e_jn.c:192:11: error: assuming 
> signed overflow does not occur when simplifying conditional to constant
> /disk/sources/4NetBSD-current/src/lib/libm/src/e_jn.c: In function 
> '__ieee754_yn':
> /disk/sources/4NetBSD-current/src/lib/libm/src/e_jn.c:236:4: error: assuming 
> signed overflow does not occur when simplifying conditional to constant
> /disk/sources/4NetBSD-current/src/lib/libm/src/e_jn.c:266:6: error: assuming 
> signed overflow does not occur when simplifying conditional to constant
> *** Error code 1
> 
> Stop.

 I have not confident that e_jn.c has no bug though...




> --apb (Alan Barrett)


-- 
-----------------------------------------------
                SAITOH Masanobu (msaitoh%execsw.org@localhost
                                 msaitoh%netbsd.org@localhost)