fix ICE out of SRA try_instantiate_multiple_fields

[Richard Guenther]

On Tue, May 6, 2008 at 12:58 PM, Olivier Hainque <hainque@adacore.com> wrote:
> Hello
>
>  Compiling the Ada testcase below on mips-irix/n32 ICEs out of an invalid
>  operation produced by the SRA try_instantiate_multiple_fields circuitry. The
>  case is extracted from the a-except runtime library unit, so the ICE presented
>  here actually breaks bootstrap for Ada on the target.
>
>     package P is
>        function Letter return String;
>     end;
>
>     package body P is
>        function X return String is
>        begin
>           return "X";
>        end;
>
>        function Letter return String is
>        begin
>           return X;
>        end;
>     end;
>
>  The compiler eventually complains about an invalid BIT_FIELD_REF conversion
>  operand, like so:
>
>     $ ../prev-gcc/gnat1 -gnatp -O1 [...] p.adb
>     ...
>     p.adb:14: error: invalid operand to unary operator
>     BIT_FIELD_REF <SR.2_4, 32, 32>;
>     p.adb:14: note: in statement
>     SR.5_6 = (unsigned int) BIT_FIELD_REF <SR.2_4, 32, 32>;
>
>  Indeed, post-gimplification NOP_EXPRs are expected to apply to "is_gimple_val"
>  operands only, and SRA turns
>
>     P.Letter ()
>     {
>     ...
>     <bb 2>:
>       D.1314 ={v} p__x ();
>       D.1313 ={v} D.1314;
>       R5b ={v} D.1313;
>       D.1315 ={v} R5b;
>       return D.1315;
>
>     }
>
>  into
>
>     P.Letter ()
>     {
>     ...
>     <bb 2>:
>       SR.1_1 = 0;
>       SR.2_2 = 0;
>       D.1314 ={v} p__x ();
>       SR.3_3 = VIEW_CONVERT_EXPR<UNSIGNED_64>(D.1314);
>       SR.2_4 = SR.3_3;
>       SR.4_5 = SR.1_1 & 0x0ffffffff00000000;
>       SR.5_6 = (unsigned int) BIT_FIELD_REF <SR.2_4, 32, 32>;
>                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>       SR.6_7 = (UNSIGNED_64) SR.5_6;
>       SR.1_8 = SR.4_5 | SR.6_7;
>       SR.7_9 = SR.1_8 & 4294967295;
>       SR.8_10 = (unsigned int) BIT_FIELD_REF <SR.2_4, 32, 0>;
>                 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>       SR.9_11 = (UNSIGNED_64) SR.8_10;
>       SR.10_12 = SR.9_11 << 32;
>       SR.1_13 = SR.7_9 | SR.10_12;
>       SR.1_14 = SR.2_4;
>       SR.11_15 = SR.1_14;
>       D.1315 ={v} VIEW_CONVERT_EXPR<struct >(SR.11_15);
>       return D.1315;
>
>  The transformed assignments initially involved Ada "fat" pointers to
>  unconstrained objects (strings), represented as a record with two regular
>  pointer fields, one to the bounds and one to the data.
>
>  IIUC, try_instantiate_multiple_fields is meant to simplify the code generated
>  for accesses to groups of bitfields. Pointer fields are usually not good
>  candidates for such coalescing, and the unexpected code expansion above I
>  think illustrates this pretty well.
>
>  Pointer fields are most of the time not coalesced by virtue of their size
>  typically being equal to that of a word, which is not the case on mips-irix
>  with the n32 ABI where the crash showed up.
>
>  The attached simple patch is a suggestion to address that by early returning
>  from try_instantiate_multiple_fields when the candidate field has
>  POINTER_TYPE.
>
>  Tested first on mips-irix/n32 by checking that the testcase doesn't ICE
>  anymore and that bootstrap proceeds past the previous failure point, then with
>  a regular bootstrap and regression test cycle on x86_64-suse-linux.
>
>  Thanks in advance,

This is ok.

Thanks,
Richard.

>  Olivier
>
>  2008-05-06  Olivier Hainque  <hainque@adacore.com>
>
>         * tree-sra.c (try_instantiate_multiple_fields): Early return
>         if field has POINTER_TYPE.
>
>         testsuite/
>         * gnat.dg/fatp_sra.adb: New test.
>
>