On Thu, May 25, 2023 at 8:50 AM Richard Biener
<[email protected]> wrote:
>
> On Wed, May 24, 2023 at 8:36 PM Alexander Monakov <[email protected]> wrote:
> >
> >
> > On Wed, 24 May 2023, Richard Biener via Gcc-patches wrote:
> >
> > > I’d have to check the ISAs what they actually do here - it of course
> > > depends
> > > on RTL semantics as well but as you say those are not strictly defined
> > > here
> > > either.
Btw, it was just noted on IRC that VSX (and maybe altivec as well)
does not adhere to this and use
just 3 bits from the shift operand for bytes and 4 for half-words.
> > Plus, we can add the following executable test to the testsuite:
>
> Yeah, that's probably a good idea. I think your documentation change
> with the added sentence about the truncation is OK. Note we have
>
> /* Shift operations for shift and rotate.
> Shift means logical shift if done on an
> unsigned type, arithmetic shift if done on a signed type.
> The second operand is the number of bits to
> shift by; it need not be the same type as the first operand and result.
> Note that the result is undefined if the second operand is larger
> than or equal to the first operand's type size.
>
> The first operand of a shift can have either an integer or a
> (non-integer) fixed-point type. We follow the ISO/IEC TR 18037:2004
> semantics for the latter.
>
> Rotates are defined for integer types only. */
> DEFTREECODE (LSHIFT_EXPR, "lshift_expr", tcc_binary, 2)
>
> in tree.def which implies short << 24 is undefined behavior (similar
> wording in generic.texi). The rtl docs say nothing about behavior
> but I think the semantics should carry over. That works for x86
> even for scalar instructions working on GPRs (masking is applied
> but fixed to 5 or 6 bits even for QImode or HImode shifts).
>
> Note that when we make these shifts well-defined there's
> also arithmetic on signed types smaller than int (which again
> doesn't exist in C) where overflow invokes undefined behavior
> in the middle-end. Unless we want to change that as well
> this is somewhat inconsistent then.
>
> There's also the issue that C 'int' is defined by INT_TYPE_SIZE
> and thus target dependent which makes what is undefined and
> what not target dependent.
>
> Richard.
>
> > #include <stdint.h>
> >
> > #define CHECK(TYPE, WIDTH, OP, COUNT, INVERT) \
> > { \
> > typedef TYPE vec __attribute__((vector_size(WIDTH))); \
> > \
> > static volatile vec zero; \
> > vec tmp = (zero-2) OP (COUNT); \
> > vec ref = INVERT zero; \
> > if (__builtin_memcmp(&tmp, &ref, sizeof tmp)) \
> > __builtin_abort(); \
> > }
> >
> > int main(void)
> > {
> > CHECK( uint8_t, 16, <<, 8, )
> > CHECK( uint8_t, 16, <<, 31, )
> > CHECK( uint8_t, 16, >>, 8, )
> > CHECK( uint8_t, 16, >>, 31, )
> > CHECK( int8_t, 16, <<, 8, )
> > CHECK( int8_t, 16, <<, 31, )
> > CHECK( int8_t, 16, >>, 8, ~)
> > CHECK( int8_t, 16, >>, 31, ~)
> > CHECK(uint16_t, 16, <<, 16, )
> > CHECK(uint16_t, 16, <<, 31, )
> > CHECK(uint16_t, 16, >>, 16, )
> > CHECK(uint16_t, 16, >>, 31, )
> > CHECK( int16_t, 16, <<, 16, )
> > CHECK( int16_t, 16, <<, 31, )
> > CHECK( int16_t, 16, >>, 16, ~)
> > CHECK( int16_t, 16, >>, 31, ~)
> > // Per-lane-variable shifts:
> > CHECK( uint8_t, 16, <<, zero+8, )
> > CHECK( uint8_t, 16, <<, zero+31, )
> > CHECK( uint8_t, 16, >>, zero+8, )
> > CHECK( uint8_t, 16, >>, zero+31, )
> > CHECK( int8_t, 16, <<, zero+8, )
> > CHECK( int8_t, 16, <<, zero+31, )
> > CHECK( int8_t, 16, >>, zero+8, ~)
> > CHECK( int8_t, 16, >>, zero+31, ~)
> > CHECK(uint16_t, 16, <<, zero+16, )
> > CHECK(uint16_t, 16, <<, zero+31, )
> > CHECK(uint16_t, 16, >>, zero+16, )
> > CHECK(uint16_t, 16, >>, zero+31, )
> > CHECK( int16_t, 16, <<, zero+16, )
> > CHECK( int16_t, 16, <<, zero+31, )
> > CHECK( int16_t, 16, >>, zero+16, ~)
> > CHECK( int16_t, 16, >>, zero+31, ~)
> >
> > // Repeat for WIDTH=32 and WIDTH=64
> > }
> >
> > Alexander
