GCC 11 Release Series
Changes, New Features, and Fixes

This page is a "brief" summary of some of the huge number of improvements in GCC 11. You may also want to check out our Porting to GCC 11 page and the full GCC documentation.

Note: GCC 11 has not been released yet, so this document is a work-in-progress.

Caveats

• The default mode for C++ is now -std=gnu++17 instead of -std=gnu++14. Note that C++17 changes to template template parameter matching can be disabled independently of other features with -fno-new-ttp-matching.
• When building GCC itself, the host compiler must now support C++11, rather than C++98. In particular bootstrapping GCC 11 using an older version of GCC requires a binary of GCC 4.8 or later, rather than of GCC 3.4 or later as was the case for bootstrapping GCC 10.
• Naming and location of auxiliary and dump output files changed. If you compile multiple input files in a single command, if you enable Link Time Optimization, or if you use -dumpbase, -dumpdir, -save-temps=*, and you expect any file other than the primary output file(s) to be created as a side effect, watch out for improvements and a few surprises. See the patch, particularly its textual description, for more details about the changes.
• -gsplit-dwarf no longer enables debug info generation on its own but requires a separate -g for this.
• The libstdc++ configure option --enable-cheaders=c_std is deprecated and will be removed in a future release. It should be possible to use --enable-cheaders=c_global (the default) with no change in behaviour.

General Improvements

• ThreadSanitizer improvements to support alternative runtimes and environments. The Linux Kernel Concurrency Sanitizer (KCSAN) is now supported.
  • Add --param tsan-distinguish-volatile to optionally emit instrumentation distinguishing volatile accesses.
  • Add --param tsan-instrument-func-entry-exit to optionally control if function entries and exits should be instrumented.

• In previous releases of GCC, the "column numbers" emitted in diagnostics were actually a count of bytes from the start of the source line. This could be problematic, both because of:

  • multibyte characters (requiring more than one byte to encode), and
  • multicolumn characters (requiring more than one column to display in a monospace font)

  For example, the character π ("GREEK SMALL LETTER PI (U+03C0)") occupies one column, and its UTF-8 encoding requires two bytes; the character 🙂 ("SLIGHTLY SMILING FACE (U+1F642)") occupies two columns, and its UTF-8 encoding requires four bytes.

  In GCC 11 the column numbers default to being column numbers, respecting multi-column characters. The old behavior can be restored using a new option -fdiagnostics-column-unit=byte. There is also a new option -fdiagnostics-column-origin=, allowing the pre-existing default of the left-hand column being column 1 to be overridden if desired (e.g. for 0-based columns). The output of -fdiagnostics-format=json has been extended to supply both byte counts and column numbers for all source locations.

  Additionally, in previous releases of GCC, tab characters in the source would be emitted verbatim when quoting source code, but be prefixed with whitespace or line number information, leading to misalignments in the resulting output when compared with the actual source. Tab characters are now printed as an appropriate number of spaces, using the -ftabstop option (which defaults to 8 spaces per tab stop).

• Introduce Hardware-assisted AddressSanitizer support. This sanitizer currently only works for the AArch64 target. It helps debug address problems similarly to AddressSanitizer but is based on partial hardware assistance and provides probabilistic protection to use less RAM at run time. Hardware-assisted AddressSanitizer is not production-ready for user space, and is provided mainly for use compiling the Linux Kernel.

  To use this sanitizer the command line arguments are:
  • -fsanitize=hwaddress to instrument userspace code.
  • -fsanitize=kernel-hwaddress to instrument kernel code.

New Languages and Language specific improvements

• GCC 11 adds support for non-rectangular loop nests in OpenMP constructs and the allocator routines of OpenMP 5.0, including initial allocate clause support in C/C++. The OMP_TARGET_OFFLOAD environment variable and the active-levels routines are now supported. For C/C++, the declare variant and map support has been extended. For Fortran, OpenMP 4.5 is now fully supported and OpenMP 5.0 support has been extended, including the following features which were before only available in C and C++: order(concurrent), device_type, memorder-clauses for flush, lastprivate with conditional modifier, atomic construct and reduction clause extensions of OpenMP 5.0, if clause with simd and cancel modifiers, target data without map clause, and limited support for the requires construct.

C family

• New attribute:
  • The no_stack_protector attribute has been added to mark functions which should not be instrumented with stack protection (-fstack-protector).
• New warnings:
  • -Wsizeof-array-div, enabled by -Wall, warns about divisions of two sizeof operators when the first one is applied to an array and the divisor does not equal the size of the array element.

• For ELF targets that support the GNU or FreeBSD OSABIs, the used attribute will now save the symbol declaration it is applied to from linker garbage collection.

  To support this behavior, used symbols that have not been placed in specific sections (e.g. with the section attribute, or the -f{function,data}-sections options) will be placed in new, unique sections.

  This functionality requires Binutils version 2.36 or later.

• A new warning -Wtsan, enabled by default, warns about unsupported features in ThreadSanitizer (currently std::atomic_thread_fence).
• A series of conditional expressions that compare the same variable can be transformed into a switch statement if each of them contains a comparison expression. Example:

          int IsHTMLWhitespace(int aChar) {
            return aChar == 0x0009 || aChar == 0x000A ||
                   aChar == 0x000C || aChar == 0x000D ||
                   aChar == 0x0020;
          }
         

  This statement can be transformed into a switch statement and then expanded into a bit-test.
• New command-line options:
  • -fbit-tests, enabled by default, can be used to enable or disable switch expansion using bit-tests.
• GCOV data file format outputs smaller files by representing zero counters in a more compact way.
• Some short options of the gcov tool have been renamed: -i to -j and -j to -H.

C++

• The default mode has been changed to -std=gnu++17.
• Several C++20 features have been implemented:
  • the compiler now supports consteval virtual functions
  • P2082R1, Fixing CTAD for aggregates
  • P0593R6, Pseudo-destructors end object lifetimes
  • P1907R1, Inconsistencies with non-type template parameters (complete implementation)
  • P1975R0, Fixing the wording of parenthesized aggregate-initialization
  • P1009R2, Array size deduction in new-expressions
  • P1099R5, using enum
  • Modules, Requires -fmodules-ts and some aspects are incomplete. Refer to C++ 20 Status
• Several C++ Defect Reports have been resolved, e.g.:
  • DR 625, Use of auto as a template-argument
  • DR 1512, Pointer comparison vs qualification conversions
  • DR 1722, Should lambda to function pointer conversion function be noexcept?
  • DR 1914, Duplicate standard attributes
  • DR 2032, Default template-arguments of variable templates
  • DR 2289, Uniqueness of decomposition declaration names
  • DR 2237, Can a template-id name a constructor?
  • DR 2303, Partial ordering and recursive variadic inheritance
  • DR 2369, Ordering between constraints and substitution
  • DR 2450, braced-init-list as a template-argument
• G++ now performs better access checking in templates (PR41437).
• reinterpret_casts in constexpr evaluation are now checked more completely (PR95307).
• The diagnostic for static_assert has been improved: the compiler now shows the expression including its template arguments (if there were any), and can point to the failing clause if the condition comprised of any logical AND operators (PR97518).
• New warnings:
  • -Wctad-maybe-unsupported, disabled by default, warns about performing class template argument deduction on a type with no deduction guides.
  • -Wrange-loop-construct, enabled by -Wall, warns when a range-based for-loop is creating unnecessary and expensive copies.
  • -Wdeprecated-enum-enum-conversion, enabled by default in C++20, warns about deprecated arithmetic conversions on operands of enumeration types, as outlined in [depr.arith.conv.enum].
  • -Wdeprecated-enum-float-conversion, enabled by default in C++20, warns about deprecated arithmetic conversions on operands where one is of enumeration type and the other is of a floating-point type, as outlined in [depr.arith.conv.enum].
  • -Wvexing-parse, enabled by default, warns about the most vexing parse rule: the cases when a declaration looks like a variable definition, but the C++ language requires it to be interpreted as a function declaration.

Runtime Library (libstdc++)

• Improved C++17 support, including:
  • std::from_chars for floating-point types.
• Improved experimental C++2a support, including:
  • Calendar additions to <chrono>.
  • std::bit_cast
  • std::source_location
  • Atomic wait and notify operations.
  • <barrier>, <latch>, and <semaphore>
  • <syncstream>
  • Efficient access to basic_stringbuf's buffer.
• Faster std::uniform_int_distribution, thanks to Daniel Lemire.

Fortran

• Added DEPRECATED to !GCC$'s attributes directive.

libgccjit

• libgccjit can now be built for MinGW
• The libgccjit API gained 10 new entry points:
  • gcc_jit_global_set_initializer
  • 9 entrypoints for directly embedding asm statements into a compile, analogous to inline asm in the C front end

New Targets and Target Specific Improvements

IA-32/x86-64

• New ISA extension support for Intel TSXLDTRK was added to GCC. TSXLDTRK intrinsics are available via the -mtsxldtrk compiler switch.
• New ISA extension support for Intel SERIALIZE was added to GCC. SERIALIZE intrinsics are available via the -mserialize compiler switch.
• New ISA extension support for Intel HRESET was added to GCC. HRESET intrinsics are available via the -mhreset compiler switch.
• New ISA extension support for Intel UINTR was added to GCC. UINTR intrinsics are available via the -muintr compiler switch.
• New ISA extension support for Intel KEYLOCKER was added to GCC. KEYLOCKER intrinsics are available via the -mkeylocker compiler switch.
• New ISA extension support for Intel AMX-TILE, AMX-INT8, AMX-BF16 was added to GCC. AMX-TILE, AMX-INT8, AMX-BF16 intrinsics are available via the -mamx-tile, -mamx-int8, -mamx-bf16 compiler switches.
• New ISA extension support for Intel AVX-VNNI was added to GCC. AVX-VNNI intrinsics are available via the -mavxvnni compiler switch.
• GCC now supports the Intel CPU named Sapphire Rapids through -march=sapphirerapids. The switch enables the MOVDIRI, MOVDIR64B, AVX512VP2INTERSECT, ENQCMD, CLDEMOTE, SERIALIZE, PTWRITE, WAITPKG, TSXLDTRK, AMT-TILE, AMX-INT8, AMX-BF16, and AVX-VNNI ISA extensions.
• GCC now supports the Intel CPU named Alderlake through -march=alderlake. The switch enables the CLDEMOTE, PTWRITE, WAITPKG, SERIALIZE, KEYLOCKER, AVX-VNNI, and HRESET ISA extensions.

Nios II

• The options -mcustom-insn=N no longer produce compiler warnings if the custom instruction is not generated due to missing optimization options such as -fno-math-errno, -ffinite-math-only, or -funsafe-math-optimizations. These warnings were not consistently emitted for all custom instructions.
• The -mcustom-fpu-cfg=fph2 option has been added to enable the custom instructions supported by the Nios II Floating Point Hardware 2 Component.

NVPTX

• The -misa default has changed from sm_30 to sm_35.
• The -m32 compiler switch has been removed.
• The -msoft-stack-reserve-local format has been fixed. Previously, it accepted -msoft-stack-reserve-local<n>. It now accepts -msoft-stack-reserve-local=<n>.

S/390, System z, IBM Z Systems

• The behavior when compiling with -fexcess-precision=standard (e.g., implied by -std=c99) on s390(x) targets can now be controlled at configure time with the flag --enable-s390-excess-float-precision. When enabled, GCC will maintain previous behavior and evaluate float expressions in double precision, which aligns with the definition of float_t as double. With the flag disabled, GCC will always evaluate float expressions in single precision. In native builds, GCC will by default match the definition of float_t in the installed glibc.

Operating Systems

Improvements to Static Analyzer

• The implementation of how program state is tracked within -fanalyzer has been completely rewritten for GCC 11, fixing numerous bugs, and allowing for the analyzer to scale to larger C source files.
• As preliminary work towards eventually supporting C++, the malloc/free checking within -fanalyzer has been generalized to also check new/delete and new[]/delete[], adding a new -Wanalyzer-mismatching-deallocation warning. However, C++ is not yet properly supported by -fanalyzer (for example, exception-handling is unimplemented).
• The analyzer has gained warnings -Wanalyzer-shift-count-negative, -Wanalyzer-shift-count-overflow, -Wanalyzer-write-to-const, and -Wanalyzer-write-to-string-literal, all enabled by default when -fanalyzer is enabled.
• The analyzer has gained new debugging options -fdump-analyzer-json and -fno-analyzer-feasibility.