FYI...if no one has anything to say, I'd like to formally post this for review.
So.... OK for trunk? Aldy On Wed, Jun 29, 2022, 11:22 Aldy Hernandez <al...@redhat.com> wrote: > Currently global ranges are stored in SSA_NAME_RANGE_INFO as a pair of > wide_int-like objects along with the nonzero bits. We frequently lose > precision when streaming out our higher resolution iranges. The plan > was always to store the full irange between passes. However, as was > originally discussed eons ago: > > https://gcc.gnu.org/pipermail/gcc-patches/2017-May/475139.html > > ...we need a memory efficient way of saving iranges, preferably using > the trailing_wide_ints idiom. > > The problem with doing so is that trailing_wide_ints assume a > compile-time specified number of elements. For irange, we need to > determine the size at run-time. > > One solution is to adapt trailing_wide_ints such that N is the maximum > number of elements allowed, and allow setting the actual number at > run-time (defaulting to N). The attached patch does this, while > requiring no changes to existing users. > > It uses a byte to store the number of elements in the > trailing_wide_ints control word. The control word is currently a > 16-bit precision, an 8-bit max-length, and the rest is used for > m_len[N]. On a 64-bit architecture, this allows for 5 elements in > m_len without having to use an extra word. With this patch, m_len[] > would be smaller by one byte (4) before consuming the padding. This > shouldn't be a problem as the only users of trailing_wide_ints use N=2 > for NUM_POLY_INT_COEFFS in aarch64, and N=3 for range_info_def. > > For irange, my plan is to use one more word to fit a maximum of 12 > elements (the above 4 plus 8 more). This would allow for 6 pairs of > sub-ranges which would be more than adequate for our needs. In > previous tests we found that 99% of ranges fit within 3-4 pairs. More > precisely, this would allow for 5 pairs, plus the nonzero bits, plus a > spare wide-int for future development. > > Ultimately this means that streaming an irange would consume one more > word than what we currently do for range_info_def. IMO this is a nice > trade-off considering we started storing a slew of wide-ints directly > ;-). > > I'm not above rolling an altogether different approach, but would > prefer to use the existing trailing infrastructure since it's mostly > what we need. > > Thoughts? > > p.s. Tested and benchmarked on x86-64 Linux. There was no discernible > performance change in our benchmark suite. > > gcc/ChangeLog: > > * wide-int.h (struct trailing_wide_ints): Add m_num_elements. > (trailing_wide_ints::set_precision): Add num_elements argument. > (trailing_wide_ints::extra_size): Same. > --- > gcc/wide-int.h | 42 +++++++++++++++++++++++++++++------------- > 1 file changed, 29 insertions(+), 13 deletions(-) > > diff --git a/gcc/wide-int.h b/gcc/wide-int.h > index 8041b6104f9..f68ccf0a0c5 100644 > --- a/gcc/wide-int.h > +++ b/gcc/wide-int.h > @@ -1373,10 +1373,13 @@ namespace wi > : public int_traits <wide_int_storage> {}; > } > > -/* An array of N wide_int-like objects that can be put at the end of > - a variable-sized structure. Use extra_size to calculate how many > - bytes beyond the sizeof need to be allocated. Use set_precision > - to initialize the structure. */ > +/* A variable-lengthed array of wide_int-like objects that can be put > + at the end of a variable-sized structure. The number of objects is > + at most N and can be set at runtime by using set_precision(). > + > + Use extra_size to calculate how many bytes beyond the > + sizeof need to be allocated. Use set_precision to initialize the > + structure. */ > template <int N> > struct GTY((user)) trailing_wide_ints > { > @@ -1387,6 +1390,9 @@ private: > /* The shared maximum length of each number. */ > unsigned char m_max_len; > > + /* The number of elements. */ > + unsigned char m_num_elements; > + > /* The current length of each number. > Avoid char array so the whole structure is not a typeless storage > that will, in turn, turn off TBAA on gimple, trees and RTL. */ > @@ -1399,12 +1405,15 @@ private: > public: > typedef WIDE_INT_REF_FOR (trailing_wide_int_storage) const_reference; > > - void set_precision (unsigned int); > + void set_precision (unsigned int precision, unsigned int num_elements = > N); > unsigned int get_precision () const { return m_precision; } > + unsigned int num_elements () const { return m_num_elements; } > trailing_wide_int operator [] (unsigned int); > const_reference operator [] (unsigned int) const; > - static size_t extra_size (unsigned int); > - size_t extra_size () const { return extra_size (m_precision); } > + static size_t extra_size (unsigned int precision, > + unsigned int num_elements = N); > + size_t extra_size () const { return extra_size (m_precision, > + m_num_elements); } > }; > > inline trailing_wide_int_storage:: > @@ -1457,11 +1466,14 @@ trailing_wide_int_storage::operator = (const T &x) > } > > /* Initialize the structure and record that all elements have precision > - PRECISION. */ > + PRECISION. NUM_ELEMENTS can be no more than N. */ > template <int N> > inline void > -trailing_wide_ints <N>::set_precision (unsigned int precision) > +trailing_wide_ints <N>::set_precision (unsigned int precision, > + unsigned int num_elements) > { > + gcc_checking_assert (num_elements <= N); > + m_num_elements = num_elements; > m_precision = precision; > m_max_len = ((precision + HOST_BITS_PER_WIDE_INT - 1) > / HOST_BITS_PER_WIDE_INT); > @@ -1484,15 +1496,19 @@ trailing_wide_ints <N>::operator [] (unsigned int > index) const > m_len[index].len, m_precision); > } > > -/* Return how many extra bytes need to be added to the end of the > structure > - in order to handle N wide_ints of precision PRECISION. */ > +/* Return how many extra bytes need to be added to the end of the > + structure in order to handle N wide_ints of precision PRECISION. > + NUM_ELEMENTS is the number of elements, or N if none specified. */ > template <int N> > inline size_t > -trailing_wide_ints <N>::extra_size (unsigned int precision) > +trailing_wide_ints <N>::extra_size (unsigned int precision, > + unsigned int num_elements) > { > unsigned int max_len = ((precision + HOST_BITS_PER_WIDE_INT - 1) > / HOST_BITS_PER_WIDE_INT); > - return (N * max_len - 1) * sizeof (HOST_WIDE_INT); > + if (num_elements > N) > + num_elements = N; > + return (num_elements * max_len - 1) * sizeof (HOST_WIDE_INT); > } > > /* This macro is used in structures that end with a trailing_wide_ints > field > -- > 2.36.1 > >
