From: Soumya AR <[email protected]> Hi,
The following are a revised set of patches that implement atomic fetch min/max in GCC. These have been revised based on discussions in this thread: https://gcc.gnu.org/pipermail/gcc-patches/2025-November/699602.html To summarize, we now define the following builtins: - __atomic_fetch_min - __atomic_fetch_max - __atomic_min_fetch - __atomic_max_fetch And resolve them to an internal function: IFN_ATOMIC_FETCH_MINMAX. If the backend does not support atomic fetch min/max operations, the IFN is lowered to a compare-and-swap loop. Otherwise, it is expanded to target-specific optabs. ---- Following discussions with Jonathan here: https://gcc.gnu.org/pipermail/gcc-patches/2026-January/706015.html I suppose implementing the op_fetch variants of the builtins is not necessary. If they are only used to support the libstdc++ operator overloading, then we can get rid of them in the compiler if we have no use of exposing these. This would also help clean up a lot of extra-handling in place to ensure the op_fetch variants of min/max are handled correctly. ---- Patch 2, ie. implementing the builtins in GCC via CAS lowering has had minor modifications since v1 of it posted here: https://gcc.gnu.org/pipermail/gcc-patches/2025-November/700787.html Patch 3 and 4 further extend this interface to expand the IFN to atomic min/max optabs for aarch64. ---- Patch 5 adds a pass that pattern-matches CAS loops implementing fetch min/max to IFN_ATOMIC_FETCH_MINMAX. This pass turned out to be a lot more complex than I was expecting so insights into its implementation would really help. I don't expect this patch to go in at Stage 4, but putting it up for feedback nonetheless, if folks get the time to look at it. ---- Soumya AR (5): libstdc++: Implement std::atomic::fetch_min/max middle-end: Add support for atomic fetch min/max builtins via CAS lowering expand: Expand IFN_ATOMIC_FETCH_MINMAX to target specific optabs aarch64: Add backend support for atomic fetch min/max operations middle-end: Pattern match CAS loops to IFN_ATOMIC_FETCH_MINMAX gcc/Makefile.in | 2 + gcc/builtins.cc | 83 +- gcc/builtins.h | 1 + gcc/c-family/c-common.cc | 58 +- gcc/config/aarch64/aarch64-protos.h | 4 + gcc/config/aarch64/aarch64.cc | 51 ++ gcc/config/aarch64/atomics.md | 54 +- gcc/config/aarch64/iterators.md | 30 +- gcc/expr.cc | 88 +- gcc/ifn-atomic-minmax-lowering.cc | 402 +++++++++ gcc/internal-fn.cc | 8 + gcc/internal-fn.def | 1 + gcc/optabs.cc | 133 +++ gcc/optabs.def | 24 + gcc/passes.def | 2 + gcc/sync-builtins.def | 15 + .../g++.dg/tree-ssa/cas-to-minmax-1.C | 23 + .../g++.dg/tree-ssa/cas-to-minmax-2.C | 23 + .../g++.dg/tree-ssa/cas-to-minmax-3.C | 21 + .../g++.dg/tree-ssa/cas-to-minmax-4.C | 22 + gcc/testsuite/gcc.dg/atomic-op-1.c | 353 ++++++++ gcc/testsuite/gcc.dg/atomic-op-2.c | 353 ++++++++ gcc/testsuite/gcc.dg/atomic-op-3.c | 353 ++++++++ gcc/testsuite/gcc.dg/atomic-op-4.c | 353 ++++++++ gcc/testsuite/gcc.dg/atomic-op-5.c | 355 ++++++++ .../gcc.dg/tree-ssa/cas-to-minmax-1.c | 18 + .../gcc.dg/tree-ssa/cas-to-minmax-2.c | 18 + .../gcc.dg/tree-ssa/cas-to-minmax-3.c | 17 + .../gcc.dg/tree-ssa/cas-to-minmax-4.c | 14 + .../gcc.dg/tree-ssa/cas-to-minmax-5.c | 19 + .../gcc.dg/tree-ssa/cas-to-minmax-6.c | 19 + .../gcc.dg/tree-ssa/cas-to-minmax-run-1.c | 75 ++ .../gcc.dg/tree-ssa/cas-to-minmax-run-2.c | 55 ++ .../gcc.target/aarch64/atomic-minmax-lse.c | 122 +++ .../gcc.target/aarch64/atomic-minmax-nolse.c | 196 +++++ .../gcc.target/aarch64/atomic-minmax.c | 128 +++ .../gcc.target/aarch64/atomic-minmax.x | 185 ++++ gcc/tree-cas-to-minmax.cc | 801 ++++++++++++++++++ gcc/tree-pass.h | 2 + libgcc/config/aarch64/lse.S | 62 +- libgcc/config/aarch64/t-lse | 3 +- libstdc++-v3/include/bits/atomic_base.h | 132 ++- libstdc++-v3/include/bits/version.def | 8 + libstdc++-v3/include/bits/version.h | 10 + libstdc++-v3/include/std/atomic | 57 ++ libstdc++-v3/src/c++23/std.cc.in | 6 + .../nonmembers_fetch_minmax.cc | 50 ++ .../atomic_ref/integral_fetch_minmax.cc | 118 +++ 48 files changed, 4823 insertions(+), 104 deletions(-) create mode 100644 gcc/ifn-atomic-minmax-lowering.cc create mode 100644 gcc/testsuite/g++.dg/tree-ssa/cas-to-minmax-1.C create mode 100644 gcc/testsuite/g++.dg/tree-ssa/cas-to-minmax-2.C create mode 100644 gcc/testsuite/g++.dg/tree-ssa/cas-to-minmax-3.C create mode 100644 gcc/testsuite/g++.dg/tree-ssa/cas-to-minmax-4.C create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/cas-to-minmax-1.c create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/cas-to-minmax-2.c create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/cas-to-minmax-3.c create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/cas-to-minmax-4.c create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/cas-to-minmax-5.c create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/cas-to-minmax-6.c create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/cas-to-minmax-run-1.c create mode 100644 gcc/testsuite/gcc.dg/tree-ssa/cas-to-minmax-run-2.c create mode 100644 gcc/testsuite/gcc.target/aarch64/atomic-minmax-lse.c create mode 100644 gcc/testsuite/gcc.target/aarch64/atomic-minmax-nolse.c create mode 100644 gcc/testsuite/gcc.target/aarch64/atomic-minmax.c create mode 100644 gcc/testsuite/gcc.target/aarch64/atomic-minmax.x create mode 100644 gcc/tree-cas-to-minmax.cc create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic_integral/nonmembers_fetch_minmax.cc create mode 100644 libstdc++-v3/testsuite/29_atomics/atomic_ref/integral_fetch_minmax.cc -- 2.43.0
