Convert microMIPS branches and jumps whose delay slot would be filled by
a generated NOP instruction to the corresponding compact form where one
exists, in a manner similar to MIPS16 JR->JRC and JALR->JALRC swap.
Do so even where the transformation switches from a 16-bit to a 32-bit
branch encoding for no benefit in code size reduction, as this is still
advantageous. This is because a branch/NOP pair takes 2 pipeline slots
or a 2-cycle completion latency except in superscalar implementations.
Whereas a compact branch may or may not stall on its target fetch, so it
will at most have a 2-cycle completion latency and may have only 1 even
in scalar implementations, and in superscalar implementations it is
expected to have no worse latency as a branch/NOP pair has. Also it
won't stall and therefore take the extra latency cycle in the not-taken
case.
Technically this is the same as MIPS16 compaction: for the qualifying
instruction encodings the APPEND_ADD_COMPACT machine code generation
method is selected where APPEND_ADD_WITH_NOP otherwise would and tells
the code generator in `append_insn' to convert the regular form of an
instruction to its corresponding compact form. For this the opcode is
tweaked as necessary and the microMIPS opcode table is scanned for the
matching updated instruction. A non-$0 `rt' operand to BEQ and BNE
instructions is moved to the `rs' operand field of BEQZC and BNEZC
encodings as required.
Unlike with MIPS16 compaction however we need to handle out-of-distance
branch relaxation as well. We do this by deferring the generation of
any delay-slot NOP required to relaxation made in `md_convert_frag', by
converting the APPEND_ADD_WITH_NOP machine code generation to APPEND_ADD
where a relaxed instruction is recorded. Relaxation then, depending on
actual code produced, chooses between either using a compact branch or
jump encoding and emitting the NOP outstanding if no compact encoding is
possible.
For code simplicity's sake the relaxation pass is retained even if the
principle of preferring a compact encoding to a 16-bit branch/NOP pair
means, in the absence of out-of-range branch relaxation, that a single
compact branch machine code instruction will eventually be produced from
a given assembly source instruction.
gas/
* config/tc-mips.c (RELAX_MICROMIPS_ENCODE): Add `nods' flag.
(RELAX_MICROMIPS_RELAX32, RELAX_MICROMIPS_TOOFAR16)
(RELAX_MICROMIPS_MARK_TOOFAR16, RELAX_MICROMIPS_CLEAR_TOOFAR16)
(RELAX_MICROMIPS_TOOFAR32, RELAX_MICROMIPS_MARK_TOOFAR32)
(RELAX_MICROMIPS_CLEAR_TOOFAR32): Shift bits.
(get_append_method): Also return APPEND_ADD_COMPACT for
microMIPS instructions.
(find_altered_mips16_opcode): Exclude macros from matching.
Factor code out...
(find_altered_opcode): ... to this new function.
(find_altered_micromips_opcode): New function.
(frag_branch_delay_slot_size): Likewise.
(append_insn): Handle microMIPS branch/jump compaction.
(macro_start): Likewise.
(relaxed_micromips_32bit_branch_length): Likewise.
(md_convert_frag): Likewise.
* testsuite/gas/mips/micromips.s: Add conditional explicit NOPs
for delay slot filling.
* testsuite/gas/mips/micromips-b16.s: Add explicit NOPs for
delay slot filling.
* testsuite/gas/mips/micromips-size-1.s: Likewise.
* testsuite/gas/mips/micromips.l: Adjust line numbers.
* testsuite/gas/mips/micromips-warn.l: Likewise.
* testsuite/gas/mips/micromips-size-1.l: Likewise.
* testsuite/gas/mips/micromips.d: Adjust padding.
* testsuite/gas/mips/micromips-trap.d: Likewise.
* testsuite/gas/mips/micromips-insn32.d: Likewise.
* testsuite/gas/mips/micromips-noinsn32.d: Likewise.
* testsuite/gas/mips/micromips@beq.d: Update patterns for
branch/jump compaction.
* testsuite/gas/mips/micromips@bge.d: Likewise.
* testsuite/gas/mips/micromips@bgeu.d: Likewise.
* testsuite/gas/mips/micromips@blt.d: Likewise.
* testsuite/gas/mips/micromips@bltu.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-4.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-4-64.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-5.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-5pic.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-5-64.d: Likewise.
* testsuite/gas/mips/micromips@branch-misc-5pic-64.d: Likewise.
* testsuite/gas/mips/micromips@jal-svr4pic-local.d: Likewise.
* testsuite/gas/mips/micromips@jal-svr4pic-local-n32.d:
Likewise.
* testsuite/gas/mips/micromips@jal-svr4pic-local-n64.d:
Likewise.
* testsuite/gas/mips/micromips@loc-swap.d: Likewise.
* testsuite/gas/mips/micromips@loc-swap-dis.d: Likewise.
* testsuite/gas/mips/micromips@relax.d: Likewise.
* testsuite/gas/mips/micromips@relax-at.d: Likewise.
* testsuite/gas/mips/micromips@relax-swap3.d: Likewise.
* testsuite/gas/mips/branch-extern-2.d: Likewise.
* testsuite/gas/mips/branch-extern-4.d: Likewise.
* testsuite/gas/mips/branch-section-2.d: Likewise.
* testsuite/gas/mips/branch-section-4.d: Likewise.
* testsuite/gas/mips/branch-weak-2.d: Likewise.
* testsuite/gas/mips/branch-weak-5.d: Likewise.
* testsuite/gas/mips/micromips-branch-absolute.d: Likewise.
* testsuite/gas/mips/micromips-branch-absolute-n32.d: Likewise.
* testsuite/gas/mips/micromips-branch-absolute-n64.d: Likewise.
* testsuite/gas/mips/micromips-branch-absolute-addend.d:
Likewise.
* testsuite/gas/mips/micromips-branch-absolute-addend-n32.d:
Likewise.
* testsuite/gas/mips/micromips-branch-absolute-addend-n64.d:
Likewise.
* testsuite/gas/mips/micromips-compact.d: New test.
* testsuite/gas/mips/mips.exp: Run the new test.
ld/
* testsuite/ld-mips-elf/micromips-branch-absolute.d: Update
patterns for branch compaction.
* testsuite/ld-mips-elf/micromips-branch-absolute-addend.d:
Likewise.
opcodes/
* micromips-opc.c (micromips_opcodes): Reorder "bc" next to "b",
"beqzc" next to "beq", "bnezc" next to "bne" and "jrc" next to
"j".
Fix a generic BFD issue with relocations against absolute symbols, which
are installed without using any individual relocation handler provided
by the backend. This causes any absolute section's addend to be lost on
REL targets such as o32 MIPS, and also relocation-specific calculation
adjustments are not made.
As an example assembling this program:
$ cat test.s
.text
foo:
b bar
b baz
.set bar, 0x1234
$ as -EB -32 -o test-o32.o test.s
$ as -EB -n32 -o test-n32.o test.s
produces this binary code:
$ objdump -dr test-o32.o test-n32.o
test-o32.o: file format elf32-tradbigmips
Disassembly of section .text:
00000000 <foo>:
0: 10000000 b 4 <foo+0x4>
0: R_MIPS_PC16 *ABS*
4: 00000000 nop
8: 1000ffff b 8 <foo+0x8>
8: R_MIPS_PC16 baz
c: 00000000 nop
test-n32.o: file format elf32-ntradbigmips
Disassembly of section .text:
00000000 <foo>:
0: 10000000 b 4 <foo+0x4>
0: R_MIPS_PC16 *ABS*+0x1230
4: 00000000 nop
8: 10000000 b c <foo+0xc>
8: R_MIPS_PC16 baz-0x4
c: 00000000 nop
$
where it is clearly visible in `test-o32.o', which uses REL relocations,
that the absolute section's addend equivalent to the value of `bar' -- a
reference to which cannot be fully resolved at the assembly time,
because the reference is PC-relative -- has been lost, as has been the
relocation-specific adjustment of -4, required to take into account the
PC+4-relative calculation made by hardware with branches and seen in the
external symbol reference to `baz' as the `ffff' addend encoded in the
instruction word. In `test-n32.o', which uses RELA relocations, the
absolute section's addend has been correctly retained.
Give precedence then in `bfd_perform_relocation' and
`bfd_install_relocation' to any individual relocation handler the
backend selected may have provided, while still resorting to the generic
calculation otherwise. This retains the semantics which we've had since
forever or before the beginning of our repository history, and is at the
very least compatible with `bfd_elf_generic_reloc' being used as the
handler.
Retain the `bfd_is_und_section' check unchanged at the beginning of
`bfd_perform_relocation' since this does not affect the semantics of the
function. The check returns the same `bfd_reloc_undefined' code the
check for a null `howto' does, so swapping the two does not matter.
Also the check is is mutually exclusive with the `bfd_is_abs_section'
check, since a section cannot be absolute and undefined both at once, so
swapping the two does not matter either.
With this change applied the program quoted above now has the in-place
addend correctly calculated and installed in the field being relocated:
$ objdump -dr fixed-o32.o
fixed-o32.o: file format elf32-tradbigmips
Disassembly of section .text:
00000000 <foo>:
0: 1000048c b 1234 <bar>
0: R_MIPS_PC16 *ABS*
4: 00000000 nop
8: 1000ffff b 8 <foo+0x8>
8: R_MIPS_PC16 baz
c: 00000000 nop
$
Add a set of MIPS tests to cover the relevant cases, including absolute
symbols with addends, and verifying that PC-relative relocations against
symbols concerned resolve to the same value in the final link regardless
of whether the REL or the RELA relocation form is used. Exclude linker
tests though which would overflow the in-place addend on REL targets and
use them as dump patterns for RELA targets only.
bfd/
* reloc.c (bfd_perform_relocation): Try the `howto' handler
first with relocations against absolute symbols.
(bfd_install_relocation): Likewise.
gas/
* testsuite/gas/mips/mips16-branch-absolute.d: Update patterns.
* testsuite/gas/mips/branch-absolute.d: New test.
* testsuite/gas/mips/branch-absolute-n32.d: New test.
* testsuite/gas/mips/branch-absolute-n64.d: New test.
* testsuite/gas/mips/branch-absolute-addend-n32.d: New test.
* testsuite/gas/mips/branch-absolute-addend-n64.d: New test.
* testsuite/gas/mips/mips16-branch-absolute-n32.d: New test.
* testsuite/gas/mips/mips16-branch-absolute-n64.d: New test.
* testsuite/gas/mips/mips16-branch-absolute-addend-n32.d: New
test.
* testsuite/gas/mips/mips16-branch-absolute-addend-n64.d: New
test.
* testsuite/gas/mips/micromips-branch-absolute.d: New test.
* testsuite/gas/mips/micromips-branch-absolute-n32.d: New test.
* testsuite/gas/mips/micromips-branch-absolute-n64.d: New test.
* testsuite/gas/mips/micromips-branch-absolute-addend-n32.d: New
test.
* testsuite/gas/mips/micromips-branch-absolute-addend-n64.d: New
test.
* testsuite/gas/mips/branch-absolute.s: New test source.
* testsuite/gas/mips/branch-absolute-addend.s: New test source.
* testsuite/gas/mips/mips16-branch-absolute-addend.s: New test
source.
* testsuite/gas/mips/micromips-branch-absolute.s: New test
source.
* testsuite/gas/mips/micromips-branch-absolute-addend.s: New
test source.
* testsuite/gas/mips/mips.exp: Run the new tests.
ld/
* testsuite/ld-mips-elf/branch-absolute.d: New test.
* testsuite/ld-mips-elf/branch-absolute-n32.d: New test.
* testsuite/ld-mips-elf/branch-absolute-n64.d: New test.
* testsuite/ld-mips-elf/branch-absolute-addend.d: New test.
* testsuite/ld-mips-elf/branch-absolute-addend-n32.d: New test.
* testsuite/ld-mips-elf/branch-absolute-addend-n64.d: New test.
* testsuite/ld-mips-elf/micromips-branch-absolute.d: New test.
* testsuite/ld-mips-elf/micromips-branch-absolute-n32.d: New
test.
* testsuite/ld-mips-elf/micromips-branch-absolute-n64.d: New
test.
* testsuite/ld-mips-elf/micromips-branch-absolute-addend.d: New
test.
* testsuite/ld-mips-elf/micromips-branch-absolute-addend-n32.d:
New test.
* testsuite/ld-mips-elf/micromips-branch-absolute-addend-n64.d:
New test.
* testsuite/ld-mips-elf/mips-elf.exp: Run the new tests, except
from `branch-absolute-addend' and
`micromips-branch-absolute-addend', referred indirectly only.
Maciej W. Rozycki