ARM64 assembler backend only accepts loads and stores with small
or aligned offset. The compiler therefore can only fold small or
aligned offsets into loads and stores. For locals and args, their
offsets to SP are not known until very late, and the compiler
makes conservative decision not folding some of them. However,
in most cases, the offset is indeed small or aligned, and can
be folded into load and store (but actually not).

This CL adds support of loads and stores with large and unaligned
offsets. When the offset doesn't fit into the instruction, it
uses two instructions and (for very large offset) the constant
pool. This way, the compiler doesn't need to be conservative,
and can simply fold the offset.

To make it work, the assembler's optab matching rules need to be
changed. Before, MOVD accepts C_UAUTO32K which matches multiple
of 8 between 0 and 32K, and also C_UAUTO16K, which may not be
multiple of 8 and does not fit into MOVD instruction. The
assembler errors in the latter case. This change makes it only
matches multiple of 8 (or offsets within ±256, which also fits
in instruction), and uses the large-or-unaligned-offset rule
for things doesn't fit (without error). Other sized move rules
are changed similarly.

Class C_UAUTO64K and C_UOREG64K are removed, as they are never
used.

In shared library, load/store of global is rewritten to using
GOT and temp register, which conflicts with the use of temp
register for assembling large offset. So the folding is disabled
for globals in shared library mode.

Reduce cmd/go binary size by 2%.

name                     old time/op    new time/op    delta
BinaryTree17-8              8.67s ± 0%     8.61s ± 0%   -0.60%  (p=0.000 n=9+10)
Fannkuch11-8                6.24s ± 0%     6.19s ± 0%   -0.83%  (p=0.000 n=10+9)
FmtFprintfEmpty-8           116ns ± 0%     116ns ± 0%     ~     (all equal)
FmtFprintfString-8          196ns ± 0%     192ns ± 0%   -1.89%  (p=0.000 n=10+10)
FmtFprintfInt-8             199ns ± 0%     198ns ± 0%   -0.35%  (p=0.001 n=9+10)
FmtFprintfIntInt-8          294ns ± 0%     293ns ± 0%   -0.34%  (p=0.000 n=8+8)
FmtFprintfPrefixedInt-8     318ns ± 1%     318ns ± 1%     ~     (p=1.000 n=10+10)
FmtFprintfFloat-8           537ns ± 0%     531ns ± 0%   -1.17%  (p=0.000 n=9+10)
FmtManyArgs-8              1.19µs ± 1%    1.18µs ± 1%   -1.41%  (p=0.001 n=10+10)
GobDecode-8                17.2ms ± 1%    17.3ms ± 2%     ~     (p=0.165 n=10+10)
GobEncode-8                14.7ms ± 1%    14.7ms ± 2%     ~     (p=0.631 n=10+10)
Gzip-8                      837ms ± 0%     836ms ± 0%   -0.14%  (p=0.006 n=9+10)
Gunzip-8                    141ms ± 0%     139ms ± 0%   -1.24%  (p=0.000 n=9+10)
HTTPClientServer-8          256µs ± 1%     253µs ± 1%   -1.35%  (p=0.000 n=10+10)
JSONEncode-8               40.1ms ± 1%    41.3ms ± 1%   +3.06%  (p=0.000 n=10+9)
JSONDecode-8                157ms ± 1%     156ms ± 1%   -0.83%  (p=0.001 n=9+8)
Mandelbrot200-8            8.94ms ± 0%    8.94ms ± 0%   +0.02%  (p=0.000 n=9+9)
GoParse-8                  8.69ms ± 0%    8.54ms ± 1%   -1.69%  (p=0.000 n=8+10)
RegexpMatchEasy0_32-8       227ns ± 1%     228ns ± 1%   +0.48%  (p=0.016 n=10+9)
RegexpMatchEasy0_1K-8      1.92µs ± 0%    1.63µs ± 0%  -15.08%  (p=0.000 n=10+9)
RegexpMatchEasy1_32-8       256ns ± 0%     251ns ± 0%   -2.19%  (p=0.000 n=10+9)
RegexpMatchEasy1_1K-8      2.38µs ± 0%    2.09µs ± 0%  -12.49%  (p=0.000 n=10+9)
RegexpMatchMedium_32-8      352ns ± 0%     354ns ± 0%   +0.39%  (p=0.002 n=10+9)
RegexpMatchMedium_1K-8      106µs ± 0%     106µs ± 0%   -0.05%  (p=0.005 n=10+9)
RegexpMatchHard_32-8       5.92µs ± 0%    5.89µs ± 0%   -0.40%  (p=0.000 n=9+8)
RegexpMatchHard_1K-8        180µs ± 0%     179µs ± 0%   -0.14%  (p=0.000 n=10+9)
Revcomp-8                   1.20s ± 0%     1.13s ± 0%   -6.29%  (p=0.000 n=9+8)
Template-8                  159ms ± 1%     154ms ± 1%   -3.14%  (p=0.000 n=9+10)
TimeParse-8                 800ns ± 3%     769ns ± 1%   -3.91%  (p=0.000 n=10+10)
TimeFormat-8                826ns ± 2%     817ns ± 2%   -1.04%  (p=0.050 n=10+10)
[Geo mean]                  145µs          143µs        -1.79%

Change-Id: I5fc42087cee9b54ea414f8ef6d6d020b80eb5985
Reviewed-on: https://go-review.googlesource.com/42172
Run-TryBot: Cherry Zhang <cherryyz@google.com>
Reviewed-by: David Chase <drchase@google.com>