[Bug target/56102] Wrong rtx cost calculated for Thumb1

[amker.cheng at gmail dot com]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=56102

--- Comment #1 from bin.cheng <amker.cheng at gmail dot com> 2013-01-25 03:46:59 UTC ---
I have investigated this issue.

GCC uses function init_lower_subreg to initialize costs of MOVE insn with
different mode, then uses this information to decompose multi-word pseudo
registers into individual registers.

The problem is ARM backend returns wrong rtx cost for SET insn with multi-word
mode. Specifically, if you define LOG_COSTS in lower-subreg.c, GCC will dump
rtx costs when compiling with:

arm-none-eabi-gcc -mthumb -mcpu=cortex-m0 -Os/-O2 ....

The dump is:
Size costs
==========

SI move: from zero cost 4, from reg cost 4
DI move: original cost 4, split cost 4 * 2
TI move: original cost 4, split cost 4 * 4
EI move: original cost 4, split cost 4 * 6
OI move: original cost 4, split cost 4 * 8
CI move: original cost 4, split cost 4 * 12
XI move: original cost 4, split cost 4 * 16
DQ move: original cost 4, split cost 4 * 2
TQ move: original cost 4, split cost 4 * 4
UDQ move: original cost 4, split cost 4 * 2
UTQ move: original cost 4, split cost 4 * 4
DA move: original cost 4, split cost 4 * 2
TA move: original cost 4, split cost 4 * 4
UDA move: original cost 4, split cost 4 * 2
UTA move: original cost 4, split cost 4 * 4
DF move: original cost 4, split cost 4 * 2
XF move: original cost 4, split cost 4 * 3
DD move: original cost 4, split cost 4 * 2
TD move: original cost 4, split cost 4 * 4
CSI move: original cost 4, split cost 4 * 2
CDI move: original cost 4, split cost 4 * 4
CTI move: original cost 4, split cost 4 * 8
CEI move: original cost 4, split cost 4 * 12
COI move: original cost 4, split cost 4 * 16
CCI move: original cost 4, split cost 4 * 24
CXI move: original cost 4, split cost 4 * 32
SC move: original cost 4, split cost 4 * 2
DC move: original cost 4, split cost 4 * 4
XC move: original cost 4, split cost 4 * 6
V8QI move: original cost 4, split cost 4 * 2
V4HI move: original cost 4, split cost 4 * 2
V2SI move: original cost 4, split cost 4 * 2
V16QI move: original cost 4, split cost 4 * 4
V8HI move: original cost 4, split cost 4 * 4
V4SI move: original cost 4, split cost 4 * 4
V2DI move: original cost 4, split cost 4 * 4
V4HF move: original cost 4, split cost 4 * 2
V2SF move: original cost 4, split cost 4 * 2
V8HF move: original cost 4, split cost 4 * 4
V4SF move: original cost 4, split cost 4 * 4
V2DF move: original cost 4, split cost 4 * 4

Speed costs
===========

SI move: from zero cost 4, from reg cost 4
DI move: original cost 4, split cost 4 * 2
TI move: original cost 4, split cost 4 * 4
EI move: original cost 4, split cost 4 * 6
OI move: original cost 4, split cost 4 * 8
CI move: original cost 4, split cost 4 * 12
XI move: original cost 4, split cost 4 * 16
DQ move: original cost 4, split cost 4 * 2
TQ move: original cost 4, split cost 4 * 4
UDQ move: original cost 4, split cost 4 * 2
UTQ move: original cost 4, split cost 4 * 4
DA move: original cost 4, split cost 4 * 2
TA move: original cost 4, split cost 4 * 4
UDA move: original cost 4, split cost 4 * 2
UTA move: original cost 4, split cost 4 * 4
DF move: original cost 4, split cost 4 * 2
XF move: original cost 4, split cost 4 * 3
DD move: original cost 4, split cost 4 * 2
TD move: original cost 4, split cost 4 * 4
CSI move: original cost 4, split cost 4 * 2
CDI move: original cost 4, split cost 4 * 4
CTI move: original cost 4, split cost 4 * 8
CEI move: original cost 4, split cost 4 * 12
COI move: original cost 4, split cost 4 * 16
CCI move: original cost 4, split cost 4 * 24
CXI move: original cost 4, split cost 4 * 32
SC move: original cost 4, split cost 4 * 2
DC move: original cost 4, split cost 4 * 4
XC move: original cost 4, split cost 4 * 6
V8QI move: original cost 4, split cost 4 * 2
V4HI move: original cost 4, split cost 4 * 2
V2SI move: original cost 4, split cost 4 * 2
V16QI move: original cost 4, split cost 4 * 4
V8HI move: original cost 4, split cost 4 * 4
V4SI move: original cost 4, split cost 4 * 4
V2DI move: original cost 4, split cost 4 * 4
V4HF move: original cost 4, split cost 4 * 2
V2SF move: original cost 4, split cost 4 * 2
V8HF move: original cost 4, split cost 4 * 4
V4SF move: original cost 4, split cost 4 * 4
V2DF move: original cost 4, split cost 4 * 4


The original MOVE insn with multi-word mode has lower costs then split insns,
thus preventing gcc from splitting.

Root cause is that thumb1_rtx_costs/thumb1_size_rtx_costs does not handle
SET/ASHIFT/ASHIFTRT/LSHIFTRT/ROTATERT patterns with multi-word mode, as
rtx_cost does.

I am working on this and will send a patch.