x86: Add -O[2|s] assembler command-line options

On x86, some instructions have alternate shorter encodings:

1. When the upper 32 bits of destination registers of

andq $imm31, %r64
testq $imm31, %r64
xorq %r64, %r64
subq %r64, %r64

known to be zero, we can encode them without the REX_W bit:

andl $imm31, %r32
testl $imm31, %r32
xorl %r32, %r32
subl %r32, %r32

This optimization is enabled with -O, -O2 and -Os.
2. Since 0xb0 mov with 32-bit destination registers zero-extends 32-bit
immediate to 64-bit destination register, we can use it to encode 64-bit
mov with 32-bit immediates.  This optimization is enabled with -O, -O2
and -Os.
3. Since the upper bits of destination registers of VEX128 and EVEX128
instructions are extended to zero, if all bits of destination registers
of AVX256 or AVX512 instructions are zero, we can use VEX128 or EVEX128
encoding to encode AVX256 or AVX512 instructions.  When 2 source
registers are identical, AVX256 and AVX512 andn and xor instructions:

VOP %reg, %reg, %dest_reg

can be encoded with

VOP128 %reg, %reg, %dest_reg

This optimization is enabled with -O2 and -Os.
4. 16-bit, 32-bit and 64-bit register tests with immediate may be
encoded as 8-bit register test with immediate.  This optimization is
enabled with -Os.

This patch does:

1. Add {nooptimize} pseudo prefix to disable instruction size
optimization.
2. Add optimize to i386_opcode_modifier to tell assembler that encoding
of an instruction may be optimized.

gas/

	PR gas/22871
	* NEWS: Mention -O[2|s].
	* config/tc-i386.c (_i386_insn): Add no_optimize.
	(optimize): New.
	(optimize_for_space): Likewise.
	(fits_in_imm7): New function.
	(fits_in_imm31): Likewise.
	(optimize_encoding): Likewise.
	(md_assemble): Call optimize_encoding to optimize encoding.
	(parse_insn): Handle {nooptimize}.
	(md_shortopts): Append "O::".
	(md_parse_option): Handle -On.
	* doc/c-i386.texi: Document -O0, -O, -O1, -O2 and -Os as well
	as {nooptimize}.
	* testsuite/gas/cfi/cfi-x86_64.d: Pass -O0 to assembler.
	* testsuite/gas/i386/ilp32/cfi/cfi-x86_64.d: Likewise.
	* testsuite/gas/i386/i386.exp: Run optimize-1, optimize-2,
	optimize-3, x86-64-optimize-1, x86-64-optimize-2,
	x86-64-optimize-3 and x86-64-optimize-4.
	* testsuite/gas/i386/optimize-1.d: New file.
	* testsuite/gas/i386/optimize-1.s: Likewise.
	* testsuite/gas/i386/optimize-2.d: Likewise.
	* testsuite/gas/i386/optimize-2.s: Likewise.
	* testsuite/gas/i386/optimize-3.d: Likewise.
	* testsuite/gas/i386/optimize-3.s: Likewise.
	* testsuite/gas/i386/x86-64-optimize-1.s: Likewise.
	* testsuite/gas/i386/x86-64-optimize-1.d: Likewise.
	* testsuite/gas/i386/x86-64-optimize-2.d: Likewise.
	* testsuite/gas/i386/x86-64-optimize-2.s: Likewise.
	* testsuite/gas/i386/x86-64-optimize-3.d: Likewise.
	* testsuite/gas/i386/x86-64-optimize-3.s: Likewise.
	* testsuite/gas/i386/x86-64-optimize-4.d: Likewise.
	* testsuite/gas/i386/x86-64-optimize-4.s: Likewise.

opcodes/

	PR gas/22871
	* i386-gen.c (opcode_modifiers): Add Optimize.
	* i386-opc.h (Optimize): New enum.
	(i386_opcode_modifier): Add optimize.
	* i386-opc.tbl: Add "Optimize" to "mov $imm, reg",
	"sub reg, reg/mem", "test $imm, acc", "test $imm, reg/mem",
	"and $imm, acc", "and $imm, reg/mem", "xor reg, reg/mem",
	"movq $imm, reg" and AVX256 and AVX512 versions of vandnps,
	vandnpd, vpandn, vpandnd, vpandnq, vxorps, vxorpd, vpxor,
	vpxord and vpxorq.
	* i386-tbl.h: Regenerated.

1 files changed, 25 insertions, 0 deletions

diff --git a/gas/doc/c-i386.texi b/gas/doc/c-i386.texi
index 6b2def0..965e0ff 100644
--- a/gas/doc/c-i386.texi
+++ b/gas/doc/c-i386.texi
@@ -411,6 +411,28 @@ with 01, 10 and 11 RC bits, respectively.
 This option specifies that the assembler should accept only AMD64 or
 Intel64 ISA in 64-bit mode.  The default is to accept both.
 
+@cindex @samp{-O0} option, i386
+@cindex @samp{-O0} option, x86-64
+@cindex @samp{-O} option, i386
+@cindex @samp{-O} option, x86-64
+@cindex @samp{-O1} option, i386
+@cindex @samp{-O1} option, x86-64
+@cindex @samp{-O2} option, i386
+@cindex @samp{-O2} option, x86-64
+@cindex @samp{-Os} option, i386
+@cindex @samp{-Os} option, x86-64
+@item -O0 | -O | -O1 | -O2 | -Os
+Optimize instruction encoding with smaller instruction size.  @samp{-O}
+and @samp{-O1} encode 64-bit register load instructions with 64-bit
+immediate as 32-bit register load instructions with 31-bit or 32-bits
+immediates and encode 64-bit register clearing instructions with 32-bit
+register clearing instructions.  @samp{-O2} includes @samp{-O1}
+optimization plus encodes 256-bit and 512-bit vector register clearing
+instructions with 128-bit vector register clearing instructions.
+@samp{-Os} includes @samp{-O2} optimization plus encodes 16-bit, 32-bit
+and 64-bit register tests with immediate as 8-bit register test with
+immediate.  @samp{-O0} turns off this optimization.
+
 @end table
 @c man end
 
@@ -647,6 +669,9 @@ Different encoding options can be specified via pseudo prefixes:
 @samp{@{rex@}} -- prefer REX prefix for integer and legacy vector
 instructions (x86-64 only).  Note that this differs from the @samp{rex}
 prefix which generates REX prefix unconditionally.
+
+@item
+@samp{@{nooptimize@}} -- disable instruction size optimization.
 @end itemize
 
 @cindex conversion instructions, i386