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

Thu Aug 20 14:27:22 PDT 2015

OK, based on testing, mingw i686 aligns long doubles to 4 bytes:

sh-4.3$ cat < a.cpp
#include <iostream>
int main() {
  struct {
    char c[1];
    long double d;
  } s;
  std::cout<<&s.c<<std::endl;
  std::cout<<&s.d<<std::endl;
}
sh-4.3$ g++ a.cpp&&./a.exe
0x28fea0
0x28fea4

I'll fix that.

2015-08-21 0:13 GMT+03:00 Richard Smith <richard at metafoo.co.uk>:

> 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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