On Thu, 25 May 2023, Richard Biener wrote:
> On Wed, May 24, 2023 at 8:36 PM Alexander Monakov <amona...@ispras.ru> 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. > > > > 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. I am no longer confident in my patch, sorry. My claim about vector shift semantics in OpenCL was wrong. In fact it specifies that RHS of a vector shift is masked to the exact bitwidth of the element type. So, to collect various angles: 1. OpenCL semantics would need an 'AND' before a shift (except VSX/Altivec). 2. From user side we had a request to follow C integer promotion semantics in https://gcc.gnu.org/PR91838 but I now doubt we can do that. 3. LLVM makes oversized vector shifts UB both for 'vector_size' and 'ext_vector_type'. 4. Vector lowering does not emit promotions, and starting from gcc-12 ranger treats oversized shifts according to the documentation you cite below, and optimizes (e.g. with '-O2 -mno-sse') typedef short v8hi __attribute__((vector_size(16))); void f(v8hi *p) { *p >>= 16; } to zeroing '*p'. If this looks unintended, I can file a bug. I still think we need to clarify semantics of vector shifts, but probably not in the way I proposed initially. What do you think? Thanks. Alexander > 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 >
