On 26.07.2016 14:51, Segher Boessenkool wrote:
On Tue, Jul 26, 2016 at 02:14:49PM +0200, Georg-Johann Lay wrote:
which returns const0_rtx because reg 18 is set in insn 43 to const0_rtx.
Total outcome is that the right shift of reg:DI 18 is transformed to a
no-op move and cancelled out in the remainder.
Why does num_sign_bit_copies return something bigger than 8?
Because it thinks the last value was const0_rtx (which is incorrect) and
Why is that incorrect? See insn 43. Is there another insn I missed?
Yes, the insn stream that matters is (prior to combine):
(insn 43 31 44 2 (set (reg:QI 18 r18)
(const_int 0 [0])) bug-combin.c:29 56 {movqi_insn}
(nil))
(insn 44 43 45 2 (set (reg:QI 19 r19 [+1 ])
(const_int 0 [0])) bug-combin.c:29 56 {movqi_insn}
(nil))
(insn 45 44 46 2 (set (reg:QI 20 r20 [+2 ])
(const_int 0 [0])) bug-combin.c:29 56 {movqi_insn}
(nil))
(insn 46 45 47 2 (set (reg:QI 21 r21 [+3 ])
(const_int 0 [0])) bug-combin.c:29 56 {movqi_insn}
(nil))
(insn 47 46 48 2 (set (reg:QI 22 r22 [+4 ])
(const_int 0 [0])) bug-combin.c:29 56 {movqi_insn}
(nil))
(insn 48 47 49 2 (set (reg:QI 23 r23 [+5 ])
(reg:QI 65 [ _3+5 ])) bug-combin.c:29 56 {movqi_insn}
(nil))
(insn 49 48 50 2 (set (reg:QI 24 r24 [+6 ])
(reg:QI 66 [ _3+6 ])) bug-combin.c:29 56 {movqi_insn}
(nil))
(insn 50 49 51 2 (set (reg:QI 25 r25 [+7 ])
(reg:QI 67 [ _3+7 ])) bug-combin.c:29 56 {movqi_insn}
(nil))
(insn 51 50 52 2 (set (reg:QI 16 r16)
(const_int 40 [0x28])) bug-combin.c:29 56 {movqi_insn}
(nil))
(insn 52 51 61 2 (set (reg:DI 18 r18)
(ashiftrt:DI (reg:DI 18 r18)
(reg:QI 16 r16))) bug-combin.c:29 1417 {ashrdi3_insn}
(expr_list:REG_DEAD (reg:QI 16 r16)
(nil)))
The SETs of insns 43...50 all overlap reg:DI 18. This is what expand does at
it breaks up a DImode move into 8 QImode moves. The first 8 instructions
comprise an &= 0xffffff0000000000.
It's corect that QI:18 is set to 0, but this does not apply to DI:18 of course.
The problem might be that get_last_value() is called for reg:DI 18 and
combine's internal bookkeeping is incorrect
struct reg_stat_type {
/* Record last point of death of (hard or pseudo) register n. */
rtx_insn *last_death;
/* Record last point of modification of (hard or pseudo) register n. */
rtx_insn *last_set;
.last_set is the set for reg:QI 18 but later insns also change hard reg:DI
18, for example the set of reg:QI 19. This means that the information in
reg_stat_type does not tell the whole story for hard regs.
One fix could be to get the mode precision of the SET from the last_set
insn and only use the information if that mode is at least as wide as the
mode of the register for which get_last_value is called. reg_stat_type has
.last_set_mode for this purpose (QImode in the test case).
Yes, last_set_mode, and there is last_set_sign_bit_copies too. Are those
correct?
(gdb) p *rsp
$9 = {last_death = 0x0, last_set = 0x7ffff730cc00, last_set_value =
0x7ffff73094f0, last_set_table_tick = 2, last_set_label = 2,
last_set_nonzero_bits = 0, last_set_sign_bit_copies = 8 '\b', last_set_mode =
QImode, last_set_invalid = 1 '\001', sign_bit_copies = 0 '\000', nonzero_bits =
0, truncation_label = 2, truncated_to_mode = DImode}
(gdb) p rsp->last_set
$10 = (rtx_insn *) 0x7ffff730cc00
(gdb) pr
(insn 43 31 44 2 (set (reg:QI 18 r18)
(const_int 0 [0])) bug-combin.c:29 56 {movqi_insn}
(nil))
(gdb) p rsp->last_set_value
$11 = (rtx) 0x7ffff73094f0
(gdb) pr
(const_int 0 [0])
The .last_set_sign_bit_copies is correct if it is applied to QImode, but the
sign bits of reg:DI 18 are contained in reg:QI 25 set by insn 50.
As already said, rsp[regno] is not used to get the number of sign bit copies
but to conclude that DI:18 is entirely set to 0, and the number of sign bit
copies is then computed for that 0.
We do have a value, and it is for this bb.
But not for the complete hard register; it's only for reg:QI 18 which is
one byte of reg:DI 18,
The rest of the hard register bits are set to unspecified values.
No, insns 44...50 set the rest of reg:DI 18 to specified values. This is what
expand does as is lowers DImode to word_mode (QImode for avr).
FYI, FIRST_PSEUDO_REGISTER is 34.
thus the code should never reach this point in the
first place and return earlier. For example:
Index: combine.c
===================================================================
--- combine.c (revision 238701)
+++ combine.c (working copy)
@@ -13206,6 +13206,13 @@ get_last_value (const_rtx x)
&& DF_INSN_LUID (rsp->last_set) >= subst_low_luid)
return 0;
+ /* If the lookup is for a hard register make sure that value contains at
least
+ as many bits as x does. */
+
+ if (regno < FIRST_PSEUDO_REGISTER
+ && GET_MODE_PRECISION (rsp->last_set_mode) < GET_MODE_PRECISION
(GET_MODE (x)))
+ return 0;
+
/* If the value has all its registers valid, return it. */
if (get_last_value_validate (&value, rsp->last_set, rsp->last_set_label,
0))
return value;
That might be a bit harsh.
In what respect?
First things first: is the information recorded correct?
I think yes, but care must be taken what may be concluded from that
information. From a set of 8 bits you cannot draw conclusion about all 64
bits; this should be obvious enough :-)
In the above case rsp[regno] holds only information for 1 sub-byte. In order
to get the complete DImode story we would have to get the info for all
sub-parts and then put them together...
Johann
Segher
