On Tue, Jul 12, 2011 at 9:34 AM, Richard Sandiford <richard.sandif...@linaro.org> wrote: > PR 48183 is caused by the fact that we don't really support integers > (or least integer constants) wider than 2*HOST_BITS_PER_WIDE_INT: > > http://gcc.gnu.org/ml/gcc-patches/2011-03/msg01220.html > > However, such constants shouldn't be needed in normal use. > They came from an unnecessary zero-initialisation of a union such as: > > union { a f1; b f2; } u = { init_f1 }; > > where f1 and f2 are the full width of the union. The zero-initialisation > gets optimised away for "real" insns, but persists in debug insns: > > http://gcc.gnu.org/ml/gcc-patches/2011-03/msg01585.html > > This patch takes up Richard's idea here: > > http://gcc.gnu.org/ml/gcc-patches/2011-03/msg01987.html > > categorize_ctor_elements currently tries to work out how many scalars a > constructor initialises (IE) and how many of those scalars are zero (ZE). > Callers can then call count_type_elements to find out how many scalars (TE) > ought to be initialised if the constructor is "complete" (i.e. if it > explicitly initialises every meaningful byte, rather than relying on > default zero-initialisation). The constructor is complete if TE == ZE, > except as noted in [A] below. > > However, count_type_elements can't return the required TE for unions, > because it would need to know which of the union's fields was initialised > by the constructor (if any). This choice of field is reflected in IE and > ZE, so would need to be reflected in TE as well. > > count_type_elements therefore punts on unions. However, the caller > can't easily tell whether it punts because of that, because of overflow, > of because of variable-sized types. > > [A] One particular case of interest is when a union constructor initialises > a field that is shorter than the union. In this case, the rest of the > union must be zeroed in order to ensure that the other fields have > predictable values. categorize_ctor_elements has a special out-parameter > to reccord this situation. > > This leads to quite a complicated interface. The patch tries to > simplify it by making categorize_ctor_elements keep track of whether > a constructor is complete. This also has the minor advantage of > avoiding double recursion: first through the constructor, > then through its type tree. > > After this change, ZE and IE are only needed when deciding how best to > implement "complete" initialisers (such as whether to do a bulk zero > initialisation anyway, and just write the nonzero elements individually). > For cases where a "leaf" constructor element is itself an aggregate with > a union, we can therefore estimate the number of scalars in the union, > and hopefully make the heuristic a bit more accurate than the current 1: > > HOST_WIDE_INT tc = count_type_elements (TREE_TYPE (value), true); > if (tc < 1) > tc = 1; > > cp/typeck2.c also wants to check whether the variable parts of a > constructor are complete. The patch uses the approach to completeness > there. This should make it a bit more general than the current code, > which only deals with non-nested constructors. > > Tested on x86_64-linux-gnu (all languages, including Ada), and on > arm-linux-gnueabi. OK to install? > > Richard > > > gcc/ > * tree.h (categorize_ctor_elements): Remove comment. Fix long line. > (count_type_elements): Delete. > (complete_ctor_at_level_p): Declare. > * expr.c (flexible_array_member_p): New function, split out from... > (count_type_elements): ...here. Make static. Replace allow_flexarr > parameter with for_ctor_p. When for_ctor_p is true, return the > number of elements that should appear in the top-level constructor, > otherwise return an estimate of the number of scalars. > (categorize_ctor_elements): Replace p_must_clear with p_complete. > (categorize_ctor_elements_1): Likewise. Use complete_ctor_at_level_p. > (complete_ctor_at_level_p): New function, borrowing union logic > from old categorize_ctor_elements_1. > (mostly_zeros_p): Return true if the constructor is not complete. > (all_zeros_p): Update call to categorize_ctor_elements. > * gimplify.c (gimplify_init_constructor): Update call to > categorize_ctor_elements. Don't call count_type_elements. > Unconditionally prevent clearing for variable-sized types, > otherwise rely on categorize_ctor_elements to detect > incomplete initializers. > > gcc/cp/ > * typeck2.c (split_nonconstant_init_1): Pass the initializer directly, > rather than a pointer to it. Return true if the whole of the value > was initialized by the generated statements. Use > complete_ctor_at_level_p instead of count_type_elements. > > gcc/testsuite/ > 2011-07-12 Chung-Lin Tang <clt...@codesourcery.com> > > * gcc.target/arm/pr48183.c: New test. >
This caused: http://gcc.gnu.org/bugzilla/show_bug.cgi?id=49736 -- H.J.
