[with CC to gcc-patches]
Original Message
Subject: [PATCH] Fix PR55876 - Make generation of paradoxical subreg in
widen_operand more robust
Date: Mon, 07 Jan 2013 11:13:49 +0100
From: Tom de Vries tom_devr...@mentor.com
To: Richard Henderson r...@redhat.com
CC: Steve Ellcey sell...@mips.com, Andrew Pinski pins...@gmail.com
Richard,
Consider test-case test.c:
...
static inline unsigned char
bar (const char *b)
{
unsigned char used = 0;
int i;
for (i = 0; i 4; ++i)
if (b[i] != 'F')
used = 1;
return used;
}
static char buffer[8];
unsigned char
foo (void)
{
return bar (buffer) ? 0 : 1;
}
...
When compiling test.c with a mips compiler, this ICE triggers:
...
$ ./install/bin/mips-linux-gnu-gcc -O3 test.c -S -mabi=64 -march=mips64
test.c: In function 'foo':
test.c:19:3: internal compiler error: in gen_rtx_SUBREG, at emit-rtl.c:776
return bar (buffer) ? 0 : 1;
...
The ICE is introduced by revision r193539 discussed at
http://gcc.gnu.org/ml/gcc-patches/2012-11/msg01148.html .
The representation of foo just before expand is this:
...
foo ()
{
unsigned charD.13 usedD.1407;
charD.2 _7;
unsigned charD.13 _13;
charD.2 _19;
charD.2 _28;
charD.2 _37;
;; basic block 2, loop depth 0
;;pred: ENTRY
# VUSE .MEM_1(D)
_19 = MEM[(const charD.2 *)bufferD.1387];
used_20 = _19 != 70 ? 1 : 0;
# VUSE .MEM_1(D)
_28 = MEM[(const charD.2 *)bufferD.1387 + 1B];
used_29 = _28 == 70 ? used_20 : 1;
# VUSE .MEM_1(D)
_37 = MEM[(const charD.2 *)bufferD.1387 + 2B];
used_38 = _37 == 70 ? used_29 : 1;
# VUSE .MEM_1(D)
_7 = MEM[(const charD.2 *)bufferD.1387 + 3B];
used_10 = _7 == 70 ? used_38 : 1;
_13 = used_10 ^ 1;
# VUSE .MEM_1(D)
return _13;
;;succ: EXIT
}
...
The used_10 operand is in a DImode reg because r193539 allows it to be promoted
while expanding
used_10 = _7 == 70 ? used_38 : 1
in expand_cond_expr_using_cmove.
The ICE happens during expansion of
_13 = used_10 ^ 1
when trying to widen the DIMode reg for use_10 from QImode to SImode:
...
#6 0x085d7da5 in widen_operand (op=0xf7cd2ec0, mode=SImode, oldmode=QImode,
unsignedp=1, no_extend=1) at
/home/vries/local/mips/upstream/src/gcc-mainline/gcc/optabs.c:333
333 return gen_rtx_SUBREG (mode, force_reg (GET_MODE (op), op), 0);
(gdb) call debug_rtx (op)
(reg:DI 222 [ used+-7 ])
...
And although the comment in widen_operand states that we're generating a
paradoxical subreg:
...
/* If MODE is no wider than a single word, we return a paradoxical
SUBREG. */
if (GET_MODE_SIZE (mode) = UNITS_PER_WORD)
return gen_rtx_SUBREG (mode, force_reg (GET_MODE (op), op), 0);
...
it's not because mode == SImode and GET_MODE (op) == DImode.
Then validate_subreg triggers the ICE in gen_rtx_SUBREG by returning false here:
...
/* For pseudo registers, we want most of the same checks. Namely:
If the register no larger than a word, the subreg must be lowpart.
If the register is larger than a word, the subreg must be the lowpart
of a subword. A subreg does *not* perform arbitrary bit extraction.
Given that we've already checked mode/offset alignment, we only have
to check subword subregs here. */
if (osize UNITS_PER_WORD
! (lra_in_progress (FLOAT_MODE_P (imode) || FLOAT_MODE_P (omode
{
enum machine_mode wmode = isize UNITS_PER_WORD ? word_mode : imode;
unsigned int low_off = subreg_lowpart_offset (omode, wmode);
if (offset % UNITS_PER_WORD != low_off)
return false;
}
...
For a valid pseudo subreg with outer mode SImode and inner mode DImode we need
the offset corresponding to the lowpart, which is 4 for -EB. But since we were
trying to generate a paradoxical subreg, the offset is 0. This explains why the
assert doesn't trigger with -EL.
Attached patch (build and tested for target mips-linux-gnu) prevents the ICE by
checking in widen_operand whether the result of the gen_rtx_SUBREG call would
indeed be a paradoxical subreg. As a consequence, it handles this case now here:
...
/* Otherwise, get an object of MODE, clobber it, and set the low-order
part to OP. */
result = gen_reg_rtx (mode);
emit_clobber (result);
emit_move_insn (gen_lowpart (GET_MODE (op), result), op);
return result;
...
So the generated code is this:
...
(insn 34 33 35 2 (clobber (reg:SI 228)) -1
(nil))
(insn 35 34 36 2 (set (subreg:DI (reg:SI 228) 0)
(reg:DI 222 [ usedD.1407+-7 ])) -1
(nil))
(insn 36 35 37 2 (set (reg:SI 229)
(xor:SI (reg:SI 228)
(const_int 1 [0x1]))) -1
...
which is correct.
I've just realized that this is probably a too conservative fix. Using this
patch:
...
Index: gcc/optabs.c
===
--- gcc/optabs.c (revision 194898)
+++ gcc/optabs.c (working copy)
@@ -327,10 +327,15 @@ widen_operand (rtx op, enum machine_mode
SUBREG_PROMOTED_UNSIGNED_P (op) == unsignedp))
return
