On Sun, Sep 26, 2021 at 10:38 PM Roger Sayle <ro...@nextmovesoftware.com> wrote:
>
>
> This patch is prototype proof-of-concept (and request for feedback)
> that touches the front-end, middle-end and backend. My recent patch to
> perform RTL constant folding of saturating arithmetic revealed how
> difficult it is to generate a (portable) test case for that functionality.
> This patch experiments with adding an "saturating" attribute to the
> C-family front-ends to set the TYPE_SATURATING flag on integer types,
> initially as a debugging/testing tool for the middle-end. GCC already
> contains logic during RTL expansion to emit [us]s_plus and [us]s_minus
> instructions via the standard named [us]ss{add,sub}<mode>3 optabs.
>
> Disappointingly, although the documentation for ssplus<mode>3 patterns
> implies this should work for arbitrary (i.e. integer) modes, the
> optab querying infrastructure (based on optabs.def) is currently
> limited to fixed-point modes. Hence the patch below contains a
> tweak to optabs.def.
>
> With both of the above pieces in place, GCC can now generate an
> ssaddsi3 instruction (such as the example provided for the nvptx
> backend), or ICE if the required saturating operation doesn't exist,
> as libgcc doesn't (yet) provide fall-back implementations for
> saturating signed and unsigned arithmetic.
>
> Sticking with the positive, the following code:
>
> typedef int sat_int32 __attribute__ ((saturating));
> int ssadd32(int x, int y) {
> sat_int32 t = (sat_int32)x + (sat_int32)y;
> return (int)t;
> }
>
> with this patch, now generates the following on nvptx-none:
>
> mov.u32 %r23, %ar0;
> mov.u32 %r24, %ar1;
> add.sat.s32 %value, %r23, %r24;
>
>
> Are any of the independent chunks below suitable for the compiler?
> Tested on nvptx-none and x86_64-pc-linux-gnu, but nothing changes
> unless __attribute__ ((saturating)) is explicitly added to the source
> code [and I'd recommend against that except for testing purposes].
>
> Eventually saturating arithmetic such as this might be useful for
> kernel security (a hot topic of last week's Linux Plumbers' Conference)
> but it would require a lot of polishing to clean-up the rough edges
> (and ideally better hardware support).
>
> Thoughts? Even if a new C-family attribute is unsuitable, is my
> logic/implementation in handle_saturating_attribute correct?
I wonder if you need to use tricks like those in handle_vector_size_attribute
to handle say
__attribute__((saturating)) int foo(void);
Now - ISTR that elsewhere Joseph suggested that taking on
saturating operations by type was eventually misguided and we should
have instead added saturating arithmetic tree codes that we could
expose via some builtin functions like the overflow ones.
Btw, I do welcome patches like this to eventually make the
types accessible to the GIMPLE frontend though we might need
something like 'stopat' to stop us from trying to expand things to
RTL when not all targets support saturating arithmetic and we
have no fallback libgcc implementation.
I think the print-tree bits are OK.
Joseph may want to chime in as to whether it's good to expose
saturating "types" more or whether that works against any intent
to retire that detail.
Richard.
>
> 2021-09-26 Roger Sayle <ro...@nextmovesoftware.com>
>
> gcc/c-family/ChangeLog
> * c-attribs (handle_saturating_attribute): New callback function
> for a "saturating" attribute to set the TYPE_SATURATING flag on
> an integer type.
> (c_common_attribute_table): New entry for "saturating".
>
> gcc/ChangeLog
> * config/nvptx/nvptx.md (ssaddsi3, sssubsi3): New define_insn
> patterns for SImode saturating addition/subtraction respectively.
>
> * optabs.def (ssadd_optab, usadd_optab, ssub_optab, usub_optab):
> Allow querying of integer modes in addition to fixed-point modes.
>
> * print-tree.c (print_node): Output "saturating" when the
> TYPE_SATURATING flag is set on integer types.
>
> Roger
> --
>
