Re: Bug#854053: coreutils: improve 2x-3x sha256sum performance on ppc64le due to current gcc optimization bug

[Michael Stone]

On Fri, Feb 03, 2017 at 11:22:28AM -0200, Gustavo Serra Scalet wrote:
> The sha256sum provided by coreutils (without openssl) is performing
> poorly with versions >= 4.9 until 7.0 (currently under development). The
> reason for that is the -fschedule-insns optimization that is used with
> -O2. By simply deactivating it, there is a performance improvement of
> 2 to 3 times.
>
> I'm attaching a patch that demonstrate that behavior but it lacks this
> condition:
> * If ppc64le
> * If gcc being used is >= 4.9 and < 7.0
>
> Notes:
> 1) gcc-7 is not affected by this bug (verified on 20170129 snapshot).
> 2) clang is not affected by this bug (verified on v3.8 and v3.9).
> 3) strangely the sha512 is not affected by this.

Sounds good in theory, just needs conditionals. :) For debian we can
just assume the compiler version and remove the patch when a newer
compiler is available as a dependency, but making it conditional on ppc 
seems essential. Upstream would probably want some kind of compiler 
based test. My understanding is that just adding this option to cflags 
when building on the appropriate architecture wouldn't work, because it 
would negatively impact other code, is that correct?

Mike Stone

(additional context quoted below)

> Below a demonstration of how it performs:
>
> ===================================================
> $ (./configure && make -j9) > /dev/null && time src/sha256sum 
> ~/ubuntu-16.10-server-ppc64el.iso
> configure: WARNING: libacl development library was not found or not usable.
> configure: WARNING: GNU coreutils will be built without ACL support.
> configure: WARNING: libattr development library was not found or not usable.
> configure: WARNING: GNU coreutils will be built without xattr support.
> configure: WARNING: libcap library was not found or not usable.
> configure: WARNING: GNU coreutils will be built without capability support.
> configure: WARNING: libgmp development library was not found or not usable.
> configure: WARNING: GNU coreutils will be built without GMP support.
> src/who.c: In function 'print_user':
> src/who.c:454:20: warning: initialization discards 'const' qualifier from 
> pointer target type [-Wdiscarded-qualifiers]
>         int   *a = utmp_ent->ut_addr_v6;
>                    ^~~~~~~~
> d14bdb413ea6cdc8d9354fcbc37a834b7de0c23f992deb0c6764d0fd5d65408e  
> /home/gut/ubuntu-16.10-server-ppc64el.iso
>
> real    0m18.670s
> user    0m16.566s
> sys     0m0.745s
>
> $ # now with the following patch:
> $ diff Makefile.in ../Makefile.in
> 8989c8989
> < @am__fastdepCC_TRUE@  $(COMPILE) -MT $@ -MD -MP -MF $$depbase.Tpo -c -o $@ $< 
> &&\
> ---
> > @am__fastdepCC_TRUE@  $(COMPILE) $$([ "$@" == "lib/sha256.o" ] && echo 
> > "-fno-schedule-insns") -MT $@ -MD -MP -MF $$depbase.Tpo -c -o $@ $< &&\
> $ cp ../Makefile.in Makefile.in
> $ (./configure && make -j9) > /dev/null && time src/sha256sum 
> ~/ubuntu-16.10-server-ppc64el.iso
> configure: WARNING: libacl development library was not found or not usable.
> configure: WARNING: GNU coreutils will be built without ACL support.
> configure: WARNING: libattr development library was not found or not usable.
> configure: WARNING: GNU coreutils will be built without xattr support.
> configure: WARNING: libcap library was not found or not usable.
> configure: WARNING: GNU coreutils will be built without capability support.
> configure: WARNING: libgmp development library was not found or not usable.
> configure: WARNING: GNU coreutils will be built without GMP support.
> src/who.c: In function 'print_user':
> src/who.c:454:20: warning: initialization discards 'const' qualifier from 
> pointer target type [-Wdiscarded-qualifiers]
>         int   *a = utmp_ent->ut_addr_v6;
>                    ^~~~~~~~
> d14bdb413ea6cdc8d9354fcbc37a834b7de0c23f992deb0c6764d0fd5d65408e  
> /home/gut/ubuntu-16.10-server-ppc64el.iso
>
> real    0m5.903s
> user    0m5.560s
> sys     0m0.255s

> --- Makefile.in 2016-11-30 16:34:55.000000000 -0200
> +++ ../Makefile.in      2017-02-03 09:33:17.936000000 -0200
> @@ -8986,7 +8986,7 @@
>
> .c.o:
> @am__fastdepCC_TRUE@    $(AM_V_CC)depbase=`echo $@ | sed 
> 's|[^/]*$$|$(DEPDIR)/&|;s|\.o$$||'`;\
> -@am__fastdepCC_TRUE@   $(COMPILE) -MT $@ -MD -MP -MF $$depbase.Tpo -c -o $@ $< 
> &&\
> +@am__fastdepCC_TRUE@  $(COMPILE) $$([ "$@" == "lib/sha256.o" ] && echo 
> "-fno-schedule-insns") -MT $@ -MD -MP -MF $$depbase.Tpo -c -o $@ $< &&\
> @am__fastdepCC_TRUE@    $(am__mv) $$depbase.Tpo $$depbase.Po
> @AMDEP_TRUE@@am__fastdepCC_FALSE@       $(AM_V_CC)source='$<' object='$@' 
> libtool=no @AMDEPBACKSLASH@
> @AMDEP_TRUE@@am__fastdepCC_FALSE@       DEPDIR=$(DEPDIR) $(CCDEPMODE) 
> $(depcomp) @AMDEPBACKSLASH@