On Fri, Oct 14, 2022 at 4:18 PM Paul Iannetta via Gcc-patches <gcc-patches@gcc.gnu.org> wrote: > > On Wed, Oct 12, 2022 at 01:18:19AM +0200, Paul Iannetta wrote: > > On Mon, Oct 10, 2022 at 11:07:06PM +0000, Joseph Myers wrote: > > > On Mon, 10 Oct 2022, Paul Iannetta via Gcc-patches wrote: > > > > > > > I have a patch to bring this feature to the C front-end as well, and > > > > would like to hear your opinion on it, especially since it may affect > > > > the feature-set of the objc front-end as well. > > > > > > > Currently, this is only a tentative patch and I did not add any tests > > > > to the testsuite. > > > > > > I think tests (possibly existing C++ tests moved to c-c++-common?) are > > > necessary to judge such a feature; it could better be judged based on > > > tests without implementation than based on implementation without tests. > > > > Currently, this feature has the following tests in g++.dg/ext/ > > - vector9.C > > - vector19.C > > - vector21.C > > - vector22.C > > - vector23.C > > - vector27.C > > - vector28.C > > provided by Marc Glisse when he implemented the feature for C++. > > > > They are all handled by my mirror implementation (after removing > > C++-only features), save for a case in vector19.C ( v ? '1' : '2', > > where v is a vector of unsigned char, but '1' and '2' are considered > > as int, which results in a type mismatch.) > > > > I'll move those tests to c-c++-common tomorrow, but will duplicate > > vector19.C and vector23.C which rely on C++-only features. > > > > During my tests, I've been using variations around this: > > > > typedef int v2si __attribute__((__vector_size__ (2 * sizeof(int)))); > > > > v2si f (v2si a, v2si b, v2si c) > > { > > v2si d = a + !b; > > v2si e = a || b; > > return c ? (a + !b) && (c - e && a) : (!!b ^ c && e); > > } > > > > It is already possible to express much of the same thing without the > > syntactic sugar but is is barely legible > > > > typedef int v2si __attribute__((__vector_size__ (2 * sizeof(int)))); > > > > v2si f (v2si a, v2si b, v2si c) > > { > > v2si d = a + (b == 0); > > v2si e = (a != 0) | (b != 0); > > return ((c != 0) & (((a + (b == 0)) != 0) & (((c - e) != 0) & (a != 0)))) > > | ((c == 0) & (((((b == 0) == 0) ^ c) != 0) & (e != 0))); > > } > > > > Paul > > I still need to check what is done by clang on the objc side, but in > order to not conflict with what was done before, a warning is > triggered by c_obj_common_truthvalue_conversion and > build_unary_operator warns if '!' is used with a vector. Both warnings > are only triggered in pedantic mode as suggested by Iain Sandoe. > > The support of the binary ops and unary ops works as the C++ front-end > does, there is however the case of the ternary conditional operator, > where the C standard mandates the promotion of the operands if they > have rank less than (unsigned) int, whereas C++ does not. > > In any case, as per the documentation of VEC_COND_EXPR, > "vec0 = vector-condition ? vec1 : vec2" is equivalent to > ``` (from tree.def) > for (int i = 0 ; i < n ; ++i) > vec0[i] = vector-condtion[i] ? vec1[i] : vec2[i]; > ``` > But this is currently not the case, even in C++ where > ``` (Ex1) > typedef signed char vec2 __attribute__((vector_size(16))); > typedef float vec2f __attribute__((vector_size( 2 * sizeof (float)))); > > void j (vec2 *x, vec2 *z, vec2f *y, vec2f *t) > { > *x = (*y < *t) ? '1' : '0'; // error: inferred scalar type ‘char’ is > // not an integer or floating-point type > // of the same size as ‘float’. > > for (int i = 0 ; i < 2 ; ++i) // fine > (*x)[i] = (*y)[i] < (*t)[i] ? '1' : '0'; // > > *z = (*x < *z) ? '1' : '0'; // fine > } > ``` > > The documentation explicitly says: > > the ternary operator ?: is available. a?b:c, where b and c are > > vectors of the same type and a is an integer vector with the same > > number of elements of the same size as b and c, computes all three > > arguments and creates a vector {a[0]?b[0]:c[0], a[1]?b[1]:c[1], …} > Here, "*y < *t" is a boolean vector (and bool is an integral type > ([basic.fundamental] 11), so this should be accepted. > > An other point is that if we look at > ``` > for (int i = 0 ; i < n ; ++i) > vec0[i] = vector-condtion[i] ? vec1[i] : vec2[i]; > ``` > implicit conversions may happen, which is completely over-looked > currently. That is, the type of (1): "v = v0 ? v1 : v2" is the lowest > common type of v, v1 and v2; and the type of (2): "v0 ? v1 : v2" is the > lowest common type of v1 and v2. (2) can appear as a parameter, but > even in that case, I think that (2) should be constrained by the type > of the parameter and we are back to case (1). > > My points are that: > - the current implementation has a bug: " *x = (*y < *t) ? '1' : > '0';" from (Ex1) should be fine. > - the current implementation does not explicetly follow the > documented behavior of VEC_COND_EXPR. > > What do you think?
Implicit promotion was explicitely not implemented for the vector extension as that would usually lead to unexpected slowness (scalar expansion). Instead I think we accept unpromoted operands when they match and diagnose other cases. ISTR this is documented somewhere but the ternary conditional operator docs may be indeed over-simplified here. The docs mention 'The operations behave like C++ @code{valarrays}.' it should probably be clarified that while v16qi a,b,c; a = b + c; computes the sum of the elements the implicit promotion/demotion required by the C/C++ standards for scalar operations is _not_ performed (and so technically considers signed vector char adds as invoking undefined behavior on overflow rather than implementation defined behavior on the truncation step - something we should mitigate by performing the adds in unsigned when C/C++ would promote) I think the non-promotion also follows what openCL does (the intent of the vector extension was to somewhat follow that spec). Richard. > > Paul > > > >