https://gcc.gnu.org/bugzilla/show_bug.cgi?id=61656
--- Comment #1 from Uroš Bizjak <ubizjak at gmail dot com> ---
(In reply to Jakub Jelinek from comment #0)
> /usr/src/gcc/obj052/gcc/xgcc -B/usr/src/gcc/obj052/gcc/
> -fno-diagnostics-show-caret -fdiagnostics-color=never -O0 -w -c -o pr4
> 2025-2.o /usr/src/gcc/gcc/testsuite/gcc.c-torture/compile/pr42025-2.c
> ../../gcc/config/i386/i386.c:6583:60: runtime error: mload of value 32669,
> which is not a valid value for type 'x86_64_reg_class'
>
> This is on passing
> typedef struct { void *p; } Ptr;
> struct A { int i; union { Ptr p; char *q; } u; };
> by value and the problem is that when processing the union with bit_offset
> 64,
> words is 1 (u is DImode 64-bit field), but when we recurse, we are called
> with
> 64-bit scalar DImode q and bit_offset 64, that is size (128-1)&0x7f and so
> it is the size < 64+64 case where we return 2 and { X86_64_INTEGER_CLASS,
> X86_64_INTEGER_CLASS }; in subclauses. But words is 1 and we merge classes
> up to num (2). A simple fix could be:
> if (!num)
> return 0;
> - for (i = 0; i < num; i++)
> + for (i = 0; i < num && i < words; i++)
> classes[i] = merge_classes (subclasses[i], classes[i]);
> in the UNION_TYPE case, as it seems the caller will not care about classes
> above returned number (words). I'd hope such a patch should not change the
> ABI even.
> I don't know whether there isn't an ABI problem related to this though, say
> if at bit_offset 64 we have just SImode field in the union rather than
> DImode,
> then I'd guess the recursive call would give us { X86_64_INTEGER_CLASS,
> X86_64_INTEGERSI_CLASS }; but we'd use X86_64_INTEGER_CLASS anyway, as we
> are looking at position 0, not 1.
X86_64_INTEGER_CLASS and X86_64_INTEGERSI_CLASS are handled in the same way,
the only difference is that the later class allows/uses cheaper movl insn. So,
if a SImode value lives at bit_offset 64, the change from INTEGER_CLASS to
INTEGERSI_CLASS does NOT introduce an ABI change - movl just narrows memory
load and sets "don't care" high bits to zero.
