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messy combine problem - PPC
- From: Zack Weinberg <zack at codesourcery dot com>
- To: gcc at gcc dot gnu dot org
- Date: Mon, 3 Dec 2001 12:49:34 -0800
- Subject: messy combine problem - PPC
I'm looking at a nasty combine problem exposed by my PPC ABI patches.
Suppose you have code like this:
struct a {
char hours, day, month;
short year;
};
extern struct a A;
extern struct a ret_struct_a();
void test(void)
{
A = ret_struct_a();
}
Right before combine, we have this RTL:
(call_insn 8 48 9 (parallel[
(set (reg:DI 3 r3)
(call (mem:SI (symbol_ref:SI ("ret_struct_a")) [0 S4 A32])
(const_int 0 [0x0])))
(use (const_int 0 [0x0]))
(clobber (scratch:SI))
] ) -1 (nil)
(expr_list:REG_UNUSED (scratch:SI)
(nil))
(nil))
(insn 9 8 11 (set (reg:SI 115)
(zero_extend:SI (subreg:HI (reg:SI 3 r3) 2)))
30 {*rs6000.md:1385}
(insn_list 8 (nil))
(expr_list:REG_DEAD (reg:SI 3 r3)
(nil)))
As far as I know this is correct RTL. Combine tries to do something
(I'm not entirely sure what) with insn 9. At -O1, in the course of
that it calls get_last_value for
(subreg:HI (reg:SI 3 r3) 2)
and get_last_value obligingly tries to return the call pattern wrapped
in a SUBREG. This blows up in simplify_gen_subreg, since the call RTX
has no mode.
Breakpoint 1, fancy_abort (
file=0x8439ba0 "../../../gcc/ppc-eabi/gcc/simplify-rtx.c", line=2649,
function=0x843a11b "simplify_gen_subreg")
at ../../../gcc/ppc-eabi/gcc/diagnostic.c:1450
1450 internal_error ("Internal compiler error in %s, at %s:%d",
(gdb) bt
#0 fancy_abort (file=0x8439ba0 "../../../gcc/ppc-eabi/gcc/simplify-rtx.c",
line=2649, function=0x843a11b "simplify_gen_subreg")
at ../../../gcc/ppc-eabi/gcc/diagnostic.c:1450
#1 0x082b61a9 in simplify_gen_subreg (outermode=HImode, op=0x40174df8,
innermode=VOIDmode, byte=0)
at ../../../gcc/ppc-eabi/gcc/simplify-rtx.c:2649
#2 0x0834d2aa in gen_lowpart_for_combine (mode=HImode, x=0x40174df8)
at ../../../gcc/ppc-eabi/gcc/combine.c:9748
#3 0x08351cc8 in get_last_value (x=0x4016ea20)
at ../../../gcc/ppc-eabi/gcc/combine.c:11366
At -O2, -fexpensive-optimizations is on, and
(zero_extend:SI (subreg:HI (reg:SI 3 r3) 2))
is replaced by
(and:SI (reg:SI 3 r3) (const_int 65535 [0xffff]))
before anything else happens; this goes through a different code path,
combine asks for the last value of (reg:SI 3 r3), and
simplify_gen_subreg is not asked to do anything with the CALL pattern.
The $64,000 question: what's the right fix here? The incoming RTL is
correct, so a fix in the machine description is inappropriate.
simplify_gen_subreg is correct to abort, (subreg:HI (call ...) N) is
ill-formed. I'm not familiar enough with combine to know where in
between these two the fix should go.
Ideas?
zw