On Wed, Aug 19, 2015 at 11:42 AM, Yaron Keren <yaron.ke...@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 <rich...@metafoo.co.uk>: > >> On Wed, Aug 19, 2015 at 10:02 AM, Yaron Keren via cfe-commits < >> cfe-commits@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@lists.llvm.org >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/cfe-commits >>> >> >>
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