r245459 - According to i686 ABI, long double size on x86 is 12 bytes not 16 bytes.

Thu Aug 20 14:13:19 PDT 2015

On Wed, Aug 19, 2015 at 11:42 AM, Yaron Keren <yaron.keren at gmail.com> wrote:

> Yes, it looks like a legacy issue. Documentation says so:
>
> *https://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/i386-and-x86-64-Options.html
> <https://gcc.gnu.org/onlinedocs/gcc-4.8.2/gcc/i386-and-x86-64-Options.html>*
>
> -m96bit-long-double-m128bit-long-doubleThese switches control the size of long
> double type. The i386 application binary interface specifies the size to
> be 96 bits, so -m96bit-long-double is the default in 32-bit mode.
>
> Modern architectures (Pentium and newer) prefer long double to be aligned
> to an 8- or 16-byte boundary. In arrays or structures conforming to the
> ABI, this is not possible. So specifying -m128bit-long-double aligns long
> double to a 16-byte boundary by padding the long double with an
> additional 32-bit zero.
>
> In the x86-64 compiler, -m128bit-long-double is the default choice as its
> ABI specifies that long double is aligned on 16-byte boundary.
>
> Notice that neither of these options enable any extra precision over the
> x87 standard of 80 bits for a long double.
>
> *Warning:* if you override the default value for your target ABI, this
> changes the size of structures and arrays containing long double variables,
> as well as modifying the function calling convention for functions taking long
> double. Hence they are not binary-compatible with code compiled without
> that switch.
>
> And practical testing agrees:
>
> sh-4.3$ cat < a.cpp
> #include <iostream>
> int main() {
>   long double a;
>   std::cout<<sizeof(a)<<std::endl;
> }
> sh-4.3$ g++ -v
> Using built-in specs.
> COLLECT_GCC=C:\mingw32\bin\g++.exe
>
> COLLECT_LTO_WRAPPER=C:/mingw32/bin/../libexec/gcc/i686-w64-mingw32/5.1.0/lto-wrapper.exe
> Target: i686-w64-mingw32
> Configured with: ../../../src/gcc-5.1.0/configure --host=i686-w64-mingw32
> --build=i686-w64-mingw32 --target=i686-w64-mingw32 --prefix=/mingw32
> --with-sysroot=/c/mingw510/i686-510-posix-dwarf-rt_v4-rev0/mingw32
> --with-gxx-include-dir=/mingw32/i686-w64-mingw32/include/c++
> --enable-shared --enable-static --disable-multilib
> --enable-languages=c,c++,fortran,objc,obj-c++,lto
> --enable-libstdcxx-time=yes --enable-threads=posix --enable-libgomp
> --enable-libatomic --enable-lto --enable-graphite --enable-checking=release
> --enable-fully-dynamic-string --enable-version-specific-runtime-libs
> --disable-sjlj-exceptions --with-dwarf2 --disable-isl-version-check
> --disable-libstdcxx-pch --disable-libstdcxx-debug --enable-bootstrap
> --disable-rpath --disable-win32-registry --disable-nls --disable-werror
> --disable-symvers --with-gnu-as --with-gnu-ld --with-arch=i686
> --with-tune=generic --with-libiconv --with-system-zlib
> --with-gmp=/c/mingw510/prerequisites/i686-w64-mingw32-static
> --with-mpfr=/c/mingw510/prerequisites/i686-w64-mingw32-static
> --with-mpc=/c/mingw510/prerequisites/i686-w64-mingw32-static
> --with-isl=/c/mingw510/prerequisites/i686-w64-mingw32-static
> --with-pkgversion='i686-posix-dwarf-rev0, Built by MinGW-W64 project'
> --with-bugurl=http://sourceforge.net/projects/mingw-w64 CFLAGS='-O2 -pipe
> -I/c/mingw510/i686-510-posix-dwarf-rt_v4-rev0/mingw32/opt/include
> -I/c/mingw510/prerequisites/i686-zlib-static/include
> -I/c/mingw510/prerequisites/i686-w64-mingw32-static/include' CXXFLAGS='-O2
> -pipe -I/c/mingw510/i686-510-posix-dwarf-rt_v4-rev0/mingw32/opt/include
> -I/c/mingw510/prerequisites/i686-zlib-static/include
> -I/c/mingw510/prerequisites/i686-w64-mingw32-static/include' CPPFLAGS=
> LDFLAGS='-pipe
> -L/c/mingw510/i686-510-posix-dwarf-rt_v4-rev0/mingw32/opt/lib
> -L/c/mingw510/prerequisites/i686-zlib-static/lib
> -L/c/mingw510/prerequisites/i686-w64-mingw32-static/lib
> -Wl,--large-address-aware'
> Thread model: posix
> gcc version 5.1.0 (i686-posix-dwarf-rev0, Built by MinGW-W64 project)
>
> sh-4.3$ g++ a.cpp
> sh-4.3$ ./a.exe
> 12
>
> Without the patch clang outputs 16 and seg faults on a boost::math example.
>

None of that answers my question. Our default GCC-compatible behavior for
x86_32 long double is to give it 4-byte alignment, 12-byte size, and this
matches the behavior I observe with GCC. Does MinGW /really/ deviate from
this and give long double a 16-byte alignment?

> 2015-08-19 21:29 GMT+03:00 Richard Smith <richard at metafoo.co.uk>:
>
>> On Wed, Aug 19, 2015 at 10:02 AM, Yaron Keren via cfe-commits <
>> cfe-commits at lists.llvm.org> wrote:
>>
>>> Author: yrnkrn
>>> Date: Wed Aug 19 12:02:32 2015
>>> New Revision: 245459
>>>
>>> URL: http://llvm.org/viewvc/llvm-project?rev=245459&view=rev
>>> Log:
>>> According to i686 ABI, long double size on x86 is 12 bytes not 16 bytes.
>>> See
>>>  https://gcc.gnu.org/onlinedocs/gcc-3.2/gcc/i386-and-x86-64-Options.html
>>>
>>>
>>> Added:
>>>     cfe/trunk/test/CodeGen/mingw-long-double-size.c
>>> Modified:
>>>     cfe/trunk/lib/Basic/Targets.cpp
>>>
>>> Modified: cfe/trunk/lib/Basic/Targets.cpp
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/lib/Basic/Targets.cpp?rev=245459&r1=245458&r2=245459&view=diff
>>>
>>> ==============================================================================
>>> --- cfe/trunk/lib/Basic/Targets.cpp (original)
>>> +++ cfe/trunk/lib/Basic/Targets.cpp Wed Aug 19 12:02:32 2015
>>> @@ -3785,7 +3785,8 @@ class MinGWX86_32TargetInfo : public Win
>>>  public:
>>>    MinGWX86_32TargetInfo(const llvm::Triple &Triple)
>>>        : WindowsX86_32TargetInfo(Triple) {
>>> -    LongDoubleWidth = LongDoubleAlign = 128;
>>> +    LongDoubleWidth = 96;
>>> +    LongDoubleAlign = 128;
>>>
>>
>> Is this really correct? It's deeply suspicious that the size is not a
>> multiple of the alignment.
>>
>>
>>>      LongDoubleFormat = &llvm::APFloat::x87DoubleExtended;
>>>    }
>>>    void getTargetDefines(const LangOptions &Opts,
>>>
>>> Added: cfe/trunk/test/CodeGen/mingw-long-double-size.c
>>> URL:
>>> http://llvm.org/viewvc/llvm-project/cfe/trunk/test/CodeGen/mingw-long-double-size.c?rev=245459&view=auto
>>>
>>> ==============================================================================
>>> --- cfe/trunk/test/CodeGen/mingw-long-double-size.c (added)
>>> +++ cfe/trunk/test/CodeGen/mingw-long-double-size.c Wed Aug 19 12:02:32
>>> 2015
>>> @@ -0,0 +1,5 @@
>>> +// RUN: %clang_cc1 -triple i686-pc-windows-gnu -S %s  -o - | FileCheck
>>> %s -check-prefix=CHECK_I686
>>> +// CHECK_I686: lda,12
>>> +// RUN: %clang_cc1 -triple x86_64-pc-windows-gnu -S %s  -o - |
>>> FileCheck %s -check-prefix=CHECK_X86_64
>>> +// CHECK_X86_64: lda,16
>>> +long double lda;
>>>
>>>
>>> _______________________________________________
>>> cfe-commits mailing list
>>> cfe-commits at lists.llvm.org
>>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits
>>>
>>
>>
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