On Sun, 12 Jan 2020 01:57:44 -0800
Mark Millard <mark...@yahoo.com> wrote:
> [So far I have not checked if there is a
> somewhat analogous C (not C++) issue or
> not for gcc9 . For C++, when registers are
> used vs. when stack space is used does not
> always match system-clang++ for g++9
> targeting 32-bit powerpc.]
>
> Building on a head -r356426 32-bit powerpc
> the following program:
>
> # more just_now.cpp
> #include <chrono>
> #include <vector>
> int main(void)
> {
> auto now0{std::chrono::steady_clock::now()};
> auto now1{std::chrono::steady_clock::now()};
> volatile std::vector<decltype(now0)> ta{ {now0,now1} };
>
> return 0;
> }
>
> via:
>
> # g++9 -std=c++17 -nostdinc++ -I/usr/include/c++/v1 \
> -pedantic -g -O2 -nodefaultlibs -lc++ -lc -lgcc_s \
> just_now.cpp
>
> produces an a.out that SIGSEGV's. Note: lack of -O?
> still fails, the code is just shorter for my presentation
> purposes when I use something like -O2 .
>
> # ldd a.out
> a.out:
> libc++.so.1 => /usr/lib/libc++.so.1 (0x41861000)
> libcxxrt.so.1 => /lib/libcxxrt.so.1 (0x4193d000)
> libc.so.7 => /lib/libc.so.7 (0x41969000)
> libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x41b74000)
>
> The same is true for powerpc-unknown-freebsd13.0-g++9 ,
> including when used via just:
>
> # /usr/local/bin/powerpc-unknown-freebsd13.0-g++9 \
> -std=c++17 -pedantic -g -O2 just_now.cpp
>
> # ldd a.out
> a.out:
> libc++.so.1 => /usr/lib/libc++.so.1 (0x41861000)
> libcxxrt.so.1 => /lib/libcxxrt.so.1 (0x4193d000)
> libm.so.5 => /lib/libm.so.5 (0x41969000)
> libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x419a4000)
> libc.so.7 => /lib/libc.so.7 (0x419cc000)
>
> ( The two g++9 variants use /usr/local/bin/ld vs.
> /usr/local/bin/powerpc-unknown-freebsd13.0-ld . )
>
> Here is the beginning of main's code from the g++9
> variants:
>
> Dump of assembler code for function main():
> 0x01800530 <+0>: stwu r1,-48(r1)
> 0x01800534 <+4>: mflr r0
> 0x01800538 <+8>: stw r0,52(r1)
> 0x0180053c <+12>: stw r30,40(r1)
> 0x01800540 <+16>: stw r31,44(r1)
> 0x01800544 <+20>: bl 0x1810d3c
> <_znst3__16chrono12steady_clock3no...@got.plt> 0x01800548
> <+24>: mr r30,r3 0x0180054c <+28>: mr r31,r4
> 0x01800550 <+32>: bl 0x1810d3c
> <_znst3__16chrono12steady_clock3no...@got.plt> . . .
>
> Note the last 2 mr instructions: the code is expecting
> now()'s time_point to be returned in registers, not
> in stack space provided by main. It is not expecting
> now() to require an address for the time_point (in r3).
>
> (That does not match the system-libraries
> implementation: that code will try to use r3
> as pointing to where to put the time_point.)
>
>
>
> By contrast, via system clang:
>
> # c++ -std=c++17 -nostdinc++ -I/usr/include/c++/v1 \
> -pedantic -g -O2 -nodefaultlibs -lc++ -lc -lgcc_s \
> just_now.cpp
>
> # ldd a.out
> a.out:
> libc++.so.1 => /usr/lib/libc++.so.1 (0x41861000)
> libcxxrt.so.1 => /lib/libcxxrt.so.1 (0x4193d000)
> libc.so.7 => /lib/libc.so.7 (0x41969000)
> libgcc_s.so.1 => /lib/libgcc_s.so.1 (0x41b74000)
>
> Dump of assembler code for function main():
> 0x01800768 <+0>: mflr r0
> 0x0180076c <+4>: stw r0,4(r1)
> 0x01800770 <+8>: stwu r1,-32(r1)
> 0x01800774 <+12>: addi r3,r1,24
> 0x01800778 <+16>: bl 0x1810aa0
> <_znst3__16chrono12steady_clock3no...@got.plt> 0x0180077c
> <+20>: addi r3,r1,16 0x01800780 <+24>: bl
> 0x1810aa0 <_znst3__16chrono12steady_clock3no...@got.plt> . . .
>
> clang++ creates stack space and passes the address
> of that stack space, with distinct values for
> each call. It does not expect to get the
> time_point from a now() from registers after the
> calls.
>
> (That does match the system-libraries
> implementation: that code will try to use r3
> as pointing to where to put the time_point.)
>
>
> This may mean that building gcc9 without
> the full bootstrap vs. with the full
> bootstrap would have consequences for the
> gcc library code and if it matched the
> FreeBSD system or not: when built by
> clang++ the code would have clang++'s
> code generation ABI properties.
>
> But even without a full bootstrap, use
> of g++9 variants (as they are) may be
> problematical for building ports and
> other code.
>
>
> For reference:
>
> # g++9 -v
> Using built-in specs.
> COLLECT_GCC=g++9
> COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc9/gcc/powerpc-portbld-freebsd13.0/9.2.0/lto-wrapper
> Target: powerpc-portbld-freebsd13.0
> Configured with:
> /wrkdirs/usr/ports/lang/gcc9/work/gcc-9.2.0/configure
> --disable-multilib --disable-plugin --disable-bootstrap --disable-nls
> --enable-gnu-indirect-function --libdir=/usr/local/lib/gcc9
> --libexecdir=/usr/local/libexec/gcc9 --program-suffix=9
> --with-as=/usr/local/bin/as --with-gmp=/usr/local
> --with-gxx-include-dir=/usr/local/lib/gcc9/include/c++/
> --with-ld=/usr/local/bin/ld --with-pkgversion='FreeBSD Ports
> Collection' --with-system-zlib --enable-languages=c,c++,objc,fortran
> --prefix=/usr/local --localstatedir=/var --mandir=/usr/local/man
> --infodir=/usr/local/share/info/gcc9
> --build=powerpc-portbld-freebsd13.0 Thread model: posix gcc version
> 9.2.0 (FreeBSD Ports Collection)
>
> # powerpc-unknown-freebsd13.0-g++9 -v
> Using built-in specs.
> COLLECT_GCC=powerpc-unknown-freebsd13.0-g++9
> COLLECT_LTO_WRAPPER=/usr/local/libexec/gcc/powerpc-unknown-freebsd13.0/9.2.0/lto-wrapper
> Target: powerpc-unknown-freebsd13.0
> Configured with:
> /wrkdirs/usr/ports/devel/freebsd-gcc9/work-powerpc/gcc-9.2.0/configure
> --target=powerpc-unknown-freebsd13.0 --disable-nls
> --enable-languages=c,c++ --enable-gnu-indirect-function
> --enable-initfini-array --program-prefix=powerpc-unknown-freebsd13.0-
> --program-suffix=9 --without-headers --with-gmp=/usr/local
> --with-pkgversion='FreeBSD Ports Collection for powerpc'
> --with-system-zlib --with-gxx-include-dir=/usr/include/c++/v1/
> --with-sysroot=/
> --with-as=/usr/local/bin/powerpc-unknown-freebsd13.0-as
> --with-ld=/usr/local/bin/powerpc-unknown-freebsd13.0-ld
> --prefix=/usr/local --localstatedir=/var --mandir=/usr/local/man
> --infodir=/usr/local/share/info/ --build=powerpc-unknown-freebsd13.0
> Thread model: posix gcc version 9.2.0 (FreeBSD Ports Collection for
> powerpc)
>
> # c++ -v
> FreeBSD clang version 9.0.1 (g...@github.com:llvm/llvm-project.git
> c1a0a213378a458fbea1a5c77b315c7dce08fd05) (based on LLVM 9.0.1)
> Target: powerpc-unknown-freebsd13.0 Thread model: posix
> InstalledDir: /usr/bin
>
>
> ===
> Mark Millard
> marklmi at yahoo.com
> ( dsl-only.net went
> away in early 2018-Mar)
>
Hi Mark,
I think this is filed as a bug against LLVM already:
https://bugs.llvm.org/show_bug.cgi?id=40736 . You can add to that PR.
- Justin
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