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From: "thomas at habets dot se" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug c/103881] New: Wconversion false positive when using |= and &= with two rvalues in binary op Date: Fri, 31 Dec 2021 16:46:46 +0000 [thread overview] Message-ID: <bug-103881-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=103881 Bug ID: 103881 Summary: Wconversion false positive when using |= and &= with two rvalues in binary op Product: gcc Version: 11.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c Assignee: unassigned at gcc dot gnu.org Reporter: thomas at habets dot se Target Milestone: --- Got: Warnings of conversion from int when doing "x |= f() | f()", with all types set to uint8_t. No warning when using "=" instead of "|=". Per https://gcc.gnu.org/bugzilla/show_bug.cgi?id=40752, this makes me wonder if "|=" creates as implicit cast to int (but why?). But if that's the case then why does "x |= f()" and "x |= a & a" not warn? And if "|=" implicitly casts to int, that should mean there's no way at all to not trigger this warning for |= into an uint8_t? Or does binary "&" implicitly cast to int (again, why?), then why does "x |= a & a" and "x = f() & f()" not warn? Reproduce program: #include<inttypes.h> uint8_t f(); void foo() { uint8_t t = 0; t |= f(); // one rvalue: no warning t |= f() & f(); // two rvalues: warn t &= f() & f(); // same with &= t = f() & f(); // no warning with assign uint8_t a = f(); // no warning t |= a & a; // no warning with variables. t |= f() & a; // warning with one rvalue t |= a & f(); // no matter position. } Output $ gcc -v -save-temps -Wconversion -c bug2.c Using built-in specs. COLLECT_GCC=gcc OFFLOAD_TARGET_NAMES=nvptx-none:amdgcn-amdhsa OFFLOAD_TARGET_DEFAULT=1 Target: x86_64-linux-gnu Configured with: ../src/configure -v --with-pkgversion='Debian 11.2.0-10' --with-bugurl=file:///usr/share/doc/gcc-11/README.Bugs --enable-languages=c,ada,c++,go,brig,d,fortran,objc,obj-c++,m2 --prefix=/usr --with-gcc-major-version-only --program-suffix=-11 --program-prefix=x86_64-linux-gnu- --enable-shared --enable-linker-build-id --libexecdir=/usr/lib --without-included-gettext --enable-threads=posix --libdir=/usr/lib --enable-nls --enable-bootstrap --enable-clocale=gnu --enable-libstdcxx-debug --enable-libstdcxx-time=yes --with-default-libstdcxx-abi=new --enable-gnu-unique-object --disable-vtable-verify --enable-plugin --enable-default-pie --with-system-zlib --enable-libphobos-checking=release --with-target-system-zlib=auto --enable-objc-gc=auto --enable-multiarch --disable-werror --enable-cet --with-arch-32=i686 --with-abi=m64 --with-multilib-list=m32,m64,mx32 --enable-multilib --with-tune=generic --enable-offload-targets=nvptx-none=/build/gcc-11-R0liEm/gcc-11-11.2.0/debian/tmp-nvptx/usr,amdgcn-amdhsa=/build/gcc-11-R0liEm/gcc-11-11.2.0/debian/tmp-gcn/usr --without-cuda-driver --enable-checking=release --build=x86_64-linux-gnu --host=x86_64-linux-gnu --target=x86_64-linux-gnu --with-build-config=bootstrap-lto-lean --enable-link-serialization=8 Thread model: posix Supported LTO compression algorithms: zlib zstd gcc version 11.2.0 (Debian 11.2.0-10) COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wconversion' '-c' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/11/cc1 -E -quiet -v -imultiarch x86_64-linux-gnu bug2.c -mtune=generic -march=x86-64 -Wconversion -fpch-preprocess -fasynchronous-unwind-tables -o bug2.i ignoring nonexistent directory "/usr/local/include/x86_64-linux-gnu" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/11/include-fixed" ignoring nonexistent directory "/usr/lib/gcc/x86_64-linux-gnu/11/../../../../x86_64-linux-gnu/include" #include "..." search starts here: #include <...> search starts here: /usr/lib/gcc/x86_64-linux-gnu/11/include /usr/local/include /usr/include/x86_64-linux-gnu /usr/include End of search list. COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wconversion' '-c' '-mtune=generic' '-march=x86-64' /usr/lib/gcc/x86_64-linux-gnu/11/cc1 -fpreprocessed bug2.i -quiet -dumpbase bug2.c -dumpbase-ext .c -mtune=generic -march=x86-64 -Wconversion -version -fasynchronous-unwind-tables -o bug2.s GNU C17 (Debian 11.2.0-10) version 11.2.0 (x86_64-linux-gnu) compiled by GNU C version 11.2.0, GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 GNU C17 (Debian 11.2.0-10) version 11.2.0 (x86_64-linux-gnu) compiled by GNU C version 11.2.0, GMP version 6.2.1, MPFR version 4.1.0, MPC version 1.2.1, isl version isl-0.24-GMP GGC heuristics: --param ggc-min-expand=100 --param ggc-min-heapsize=131072 Compiler executable checksum: 010de8893e7b0de1cd6903c6a1890e7a bug2.c: In function ‘foo’: bug2.c:7:8: warning: conversion from ‘int’ to ‘uint8_t’ {aka ‘unsigned char’} may change value [-Wconversion] 7 | t |= f() & f(); // two rvalues: warn | ^ bug2.c:8:8: warning: conversion from ‘int’ to ‘uint8_t’ {aka ‘unsigned char’} may change value [-Wconversion] 8 | t &= f() & f(); // same with &= | ^ bug2.c:13:8: warning: conversion from ‘int’ to ‘uint8_t’ {aka ‘unsigned char’} may change value [-Wconversion] 13 | t |= f() & a; // warning with one rvalue | ^ bug2.c:14:8: warning: conversion from ‘int’ to ‘uint8_t’ {aka ‘unsigned char’} may change value [-Wconversion] 14 | t |= a & f(); // no matter position. | ^ COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wconversion' '-c' '-mtune=generic' '-march=x86-64' as -v --64 -o bug2.o bug2.s GNU assembler version 2.37 (x86_64-linux-gnu) using BFD version (GNU Binutils for Debian) 2.37 COMPILER_PATH=/usr/lib/gcc/x86_64-linux-gnu/11/:/usr/lib/gcc/x86_64-linux-gnu/11/:/usr/lib/gcc/x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/11/:/usr/lib/gcc/x86_64-linux-gnu/ LIBRARY_PATH=/usr/lib/gcc/x86_64-linux-gnu/11/:/usr/lib/gcc/x86_64-linux-gnu/11/../../../x86_64-linux-gnu/:/usr/lib/gcc/x86_64-linux-gnu/11/../../../../lib/:/lib/x86_64-linux-gnu/:/lib/../lib/:/usr/lib/x86_64-linux-gnu/:/usr/lib/../lib/:/usr/lib/gcc/x86_64-linux-gnu/11/../../../:/lib/:/usr/lib/ COLLECT_GCC_OPTIONS='-v' '-save-temps' '-Wconversion' '-c' '-mtune=generic' '-march=x86-64'
next reply other threads:[~2021-12-31 16:46 UTC|newest] Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top 2021-12-31 16:46 thomas at habets dot se [this message] 2021-12-31 17:36 ` [Bug c/103881] " thomas at habets dot se 2021-12-31 17:48 ` jakub at gcc dot gnu.org 2022-01-02 12:35 ` thomas at habets dot se 2022-01-11 18:15 ` cvs-commit at gcc dot gnu.org 2022-01-20 3:58 ` egallager at gcc dot gnu.org 2022-01-20 14:56 ` thomas at habets dot se 2022-01-24 9:21 ` cvs-commit at gcc dot gnu.org 2022-05-10 8:22 ` cvs-commit at gcc dot gnu.org 2022-05-10 15:31 ` jakub at gcc dot gnu.org
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