Re: [Committed] PR17959: Use paradoxical SUBREGs in emit_group_store

[Eric Botcazou <ebotcazou at libertysurf dot fr>]

> Instead of initializing the desination to zero, and subsequently
> filling the upper and lower parts, the patch below tweaks expr.c's
> emit_group_store to identify which incoming register will form
> the lowpart of the result, and then uses a paradoxical SUBREG to
> initialize the destination, and the fills the uppers parts as before.

This appears to badly interact with the usual PARALLEL idiom:

(gdb) p debug_rtx(src)
(parallel:DI [
        (expr_list:REG_DEP_TRUE (reg:SF 32 %f0)
            (const_int 0 [0x0]))
    ])

(gdb) p debug_rtx(dst)
(reg:DI 107 [ D.2717 ])

used to move the contents of (reg:SF 32 %f0) to the high part of (reg:DI 107) 
on SPARC, now it moves it to the low part.

In particular, the test

	  bytepos = subreg_lowpart_offset (outer, inner);
	  if (INTVAL (XEXP (XVECEXP (src, 0, start), 1)) == bytepos)

looks suspicious to me since 'bytes' is always 0 for a paradoxical subreg.


That's the cause of the following failures on SPARC 64-bit:

FAIL: tmpdir-gcc.dg-struct-layout-1/t026 c_compat_x_tst.o-c_compat_y_tst.o 
execute 
FAIL: tmpdir-gcc.dg-struct-layout-1/t027 c_compat_x_tst.o-c_compat_y_tst.o 
execute 

Testcase attached.

-- 
Eric Botcazou

struct S2439 { unsigned long long int a[0];float b; };
struct S2439 s2439;
extern struct S2439 a2439[5];
extern struct S2439 check2439 (struct S2439, struct S2439 *, struct S2439);
extern void check2439va (int i, ...);
extern void checkx2439 (struct S2439);

void test2439 (void)
{
  s2439.b = -41742.765625;
  checkx2439 (check2439 (s2439, &a2439[1], a2439[2]));
}