Re: [PATCHv3] sysdeps/ieee754: prevent maybe-uninitialized errors with -O [BZ #19444]

[Martin Jansa <martin dot jansa at gmail dot com>]

On Sun, Sep 30, 2018 at 07:57:11PM +0200, Martin Jansa wrote:
> On Sun, Sep 30, 2018 at 04:52:56PM +0000, Joseph Myers wrote:
> > On Sun, 30 Sep 2018, Martin Jansa wrote:
> > 
> > > +	    /* With GCC 8 (and older) when compiling with -O the compiler
> > > +	       warns that the variable 'temp', may be used uninitialized.
> > > +	       The switch above covers all possible values of n & 3
> > > +	       but GCC without VRP enabled isn't able to figure out the
> > > +	       range of possible values is [0,3] as explained in:
> > > +	       https://gcc.gnu.org/bugzilla/show_bug.cgi?id=69230
> > > +	       so it's false possitive with -O1 and lower. */
> > 
> > "positive" (for all such comments).  Also note there should be two spaces 
> > at the end of a comment between "." and the trailing "*/".
> 
> OK, will update.
> 
> > Are you sure about the "and lower", or is this warning disabled for -O0?
> 
> I'm pretty sure that -ftree-vrp isn't enabled with -O0, but will retry
> if it also shows this warning with -O0.

You were right, it warning isn't triggered with -O0, I'll drop the " and
lower" part in v4.

docker-shr @ ~/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/git/math $ x86_64-oe-linux-gcc  -m64 -march=core2 -mtune=core2 -msse3 -mfpmath=sse  --sysroot=/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot ../sysdeps/ieee754/ldbl-96/e_jnl.c -c -std=gnu11 -fgnu89-inline  -O1 -pipe -g -feliminate-unused-debug-types -fdebug-prefix-map=/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0=/usr/src/debug/glibc/2.28-r0 -fdebug-prefix-map=/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot= -fdebug-prefix-map=/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot-native= -Wall -Wwrite-strings -Wundef -Werror -fmerge-all-constants -frounding-math -fno-stack-protector -Wstrict-prototypes -Wold-style-definition -fno-math-errno           -D__NO_MATH_INLINES -D__LIBC_INTERNAL_MATH_INLINES -I../include -I/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux/math  -I/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux  -I../sysdeps/unix/sysv/linux/x86_64/64  -I../sysdeps/unix/sysv/linux/x86_64  -I../sysdeps/unix/sysv/linux/x86/include -I../sysdeps/unix/sysv/linux/x86  -I../sysdeps/x86/nptl  -I../sysdeps/unix/sysv/linux/wordsize-64  -I../sysdeps/x86_64/nptl  -I../sysdeps/unix/sysv/linux/include -I../sysdeps/unix/sysv/linux  -I../sysdeps/nptl  -I../sysdeps/pthread  -I../sysdeps/gnu  -I../sysdeps/unix/inet  -I../sysdeps/unix/sysv  -I../sysdeps/unix/x86_64  -I../sysdeps/unix  -I../sysdeps/posix  -I../sysdeps/x86_64/64  -I../sysdeps/x86_64/fpu/multiarch  -I../sysdeps/x86_64/fpu  -I../sysdeps/x86/fpu/include -I../sysdeps/x86/fpu  -I../sysdeps/x86_64/multiarch  -I../sysdeps/x86_64  -I../sysdeps/x86  -I../sysdeps/ieee754/float128  -I../sysdeps/ieee754/ldbl-96/include -I../sysdeps/ieee754/ldbl-96  -I../sysdeps/ieee754/dbl-64/wordsize-64  -I../sysdeps/ieee754/dbl-64  -I../sysdeps/ieee754/flt-32  -I../sysdeps/wordsize-64  -I../sysdeps/ieee754  -I../sysdeps/generic  -I.. -I../libio -I. -nostdinc -isystem /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot-native/usr/bin/x86_64-oe-linux.gcc-cross-initial-x86_64/../../lib/x86_64-oe-linux.gcc-cross-initial-x86_64/gcc/x86_64-oe-linux/8.2.0/include -isystem /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot-native/usr/bin/x86_64-oe-linux.gcc-cross-initial-x86_64/../../lib/x86_64-oe-linux.gcc-cross-initial-x86_64/gcc/x86_64-oe-linux/8.2.0/include-fixed -isystem /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot/usr/include  -D_LIBC_REENTRANT -include /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux/libc-modules.h -DMODULE_NAME=libm -include ../include/libc-symbols.h       -DTOP_NAMESPACE=glibc -I../soft-fp -o /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux/math/e_jnl.o -MD -MP -MF /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux/math/e_jnl.o.dt -MT /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux/math/e_jnl.o
../sysdeps/ieee754/ldbl-96/e_jnl.c: In function '__ieee754_jnl':
../sysdeps/ieee754/ldbl-96/e_jnl.c:147:20: error: 'temp' may be used uninitialized in this function [-Werror=maybe-uninitialized]
      b = invsqrtpi * temp / sqrtl (x);
          ~~~~~~~~~~^~~~~~
../sysdeps/ieee754/ldbl-96/e_jnl.c: In function '__ieee754_ynl':
../sysdeps/ieee754/ldbl-96/e_jnl.c:376:16: error: 'temp' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  b = invsqrtpi * temp / sqrtl (x);
      ~~~~~~~~~~^~~~~~
cc1: all warnings being treated as errors
docker-shr @ ~/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/git/math $ x86_64-oe-linux-gcc  -m64 -march=core2 -mtune=core2 -msse3 -mfpmath=sse  --sysroot=/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot ../sysdeps/ieee754/ldbl-96/e_jnl.c -c -std=gnu11 -fgnu89-inline  -O0 -pipe -g -feliminate-unused-debug-types -fdebug-prefix-map=/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0=/usr/src/debug/glibc/2.28-r0 -fdebug-prefix-map=/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot= -fdebug-prefix-map=/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot-native= -Wall -Wwrite-strings -Wundef -Werror -fmerge-all-constants -frounding-math -fno-stack-protector -Wstrict-prototypes -Wold-style-definition -fno-math-errno           -D__NO_MATH_INLINES -D__LIBC_INTERNAL_MATH_INLINES -I../include -I/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux/math  -I/OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux  -I../sysdeps/unix/sysv/linux/x86_64/64  -I../sysdeps/unix/sysv/linux/x86_64  -I../sysdeps/unix/sysv/linux/x86/include -I../sysdeps/unix/sysv/linux/x86  -I../sysdeps/x86/nptl  -I../sysdeps/unix/sysv/linux/wordsize-64  -I../sysdeps/x86_64/nptl  -I../sysdeps/unix/sysv/linux/include -I../sysdeps/unix/sysv/linux  -I../sysdeps/nptl  -I../sysdeps/pthread  -I../sysdeps/gnu  -I../sysdeps/unix/inet  -I../sysdeps/unix/sysv  -I../sysdeps/unix/x86_64  -I../sysdeps/unix  -I../sysdeps/posix  -I../sysdeps/x86_64/64  -I../sysdeps/x86_64/fpu/multiarch  -I../sysdeps/x86_64/fpu  -I../sysdeps/x86/fpu/include -I../sysdeps/x86/fpu  -I../sysdeps/x86_64/multiarch  -I../sysdeps/x86_64  -I../sysdeps/x86  -I../sysdeps/ieee754/float128  -I../sysdeps/ieee754/ldbl-96/include -I../sysdeps/ieee754/ldbl-96  -I../sysdeps/ieee754/dbl-64/wordsize-64  -I../sysdeps/ieee754/dbl-64  -I../sysdeps/ieee754/flt-32  -I../sysdeps/wordsize-64  -I../sysdeps/ieee754  -I../sysdeps/generic  -I.. -I../libio -I. -nostdinc -isystem /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot-native/usr/bin/x86_64-oe-linux.gcc-cross-initial-x86_64/../../lib/x86_64-oe-linux.gcc-cross-initial-x86_64/gcc/x86_64-oe-linux/8.2.0/include -isystem /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot-native/usr/bin/x86_64-oe-linux.gcc-cross-initial-x86_64/../../lib/x86_64-oe-linux.gcc-cross-initial-x86_64/gcc/x86_64-oe-linux/8.2.0/include-fixed -isystem /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/recipe-sysroot/usr/include  -D_LIBC_REENTRANT -include /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux/libc-modules.h -DMODULE_NAME=libm -include ../include/libc-symbols.h       -DTOP_NAMESPACE=glibc -I../soft-fp -o /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux/math/e_jnl.o -MD -MP -MF /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux/math/e_jnl.o.dt -MT /OE/build/oe-core/tmp-glibc/work/core2-64-oe-linux/glibc/2.28-r0/build-x86_64-oe-linux/math/e_jnl.o

> > (glibc disallows building with -O0 at present, but I think we *should* fix 
> > things to be able to build with -O0, because being able to build with -O0 
> > would be helpful for debugging - any particular pieces needing 
> > optimization should then just have it force-enabled, or functions declared 
> > as always_inline, or whatever is needed, locally.)
> 
> I agree, but I'm not volunteering to fix all of -O0 issues reported in
> [BZ #19444], my motivation was just to fix debug builds in Yocto which
> is by default using -O (and issues with -Os which I'm using for some of
> my builds):
> http://lists.openembedded.org/pipermail/openembedded-core/2018-September/155693.html
> 
> Fixing:
> https://sourceware.org/glibc/wiki/FAQ#Why_do_I_get:.60.23error_.22glibc_cannot_be_compiled_without_optimization.22.27.2C_when_trying_to_compile_GNU_libc_with_GNU_CC.3F
> is out of my league and time I'm willing to spend, I'll leave it to
> someone more knowledgable about glibc internals.
> 
> Cheers,
> -- 
> Martin 'JaMa' Jansa     jabber: Martin.Jansa@gmail.com



-- 
Martin 'JaMa' Jansa     jabber: Martin.Jansa@gmail.com

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