On Thu, 19 Sep 2024, mmalcom...@nvidia.com wrote: > 6) Anything special about floating point maths that I'm tripping up on?
Correct atomic operations with floating-point operands should ensure that exceptions raised exactly correspond to the operands for which the operation succeeded, and not to the operands for any previous attempts where the compare-exchange failed. There is a lengthy note in the C standard (in C11 it's a footnote in 6.5.16.2, in C17 it's a Note in 6.5.16.2 and in C23 that subclause has become 6.5.17.3) that discusses appropriate code sequences to achieve this. In GCC the implementation of this is in c-typeck.cc:build_atomic_assign, which in turn calls targetm.atomic_assign_expand_fenv (note that we have the complication for C of not introducing libm dependencies in code that only uses standard language features and not <math.h>, <fenv.h> or <complex.h>, so direct use of <fenv.h> functions is inappropriate here). I would expect such built-in functions to follow the same semantics for floating-point exceptions as _Atomic compound assignment does. (Note that _Atomic compound assignment is more general in the allowed operands, because compound assignment is a heterogeneous operation - for example, the special floating-point logic in build_atomic_assign includes the case where the LHS of the compound assignment is of atomic integer type but the RHS is of floating type. However, built-in functions allow memory orders other than seq_cst to be used, whereas _Atomic compound assignment is limited to the seq_cst case.) So it would seem appropriate for the implementation of such built-in functions to make use of targetm.atomic_assign_expand_fenv for floating-point environment handling, and for testcases to include tests analogous to c11-atomic-exec-5.c that exceptions are being handled correctly. Cf. N2329 which suggested such operations for C in <stdatomic.h> (but tried to do to many things in one paper to be accepted into C); it didn't go into the floating-point exceptions semantics but simple correctness would indicate avoiding spurious exceptions from discarded computations. -- Joseph S. Myers josmy...@redhat.com
