[MC] Decrease direct access to getContents()

to make it easier to store fragment contents out-of-line. Loosely based
on Alexis Engelke's prototype.

[MC] Add .loc_label instruction (#99710)

As discussed in [the
RFC](https://discourse.llvm.org/t/rfc-extending-llvm-mc-loc-directive-with-labeling-support/79608)
we need a way to create labels in t

[MC] Add .loc_label instruction (#99710)

As discussed in [the
RFC](https://discourse.llvm.org/t/rfc-extending-llvm-mc-loc-directive-with-labeling-support/79608)
we need a way to create labels in the assembler-generated line section
in order to support the future addition of the
[DW_AT_LLVM_stmt_sequence](https://discourse.llvm.org/t/rfc-new-dwarf-attribute-for-symbolication-of-merged-functions/79434)
attribute.

We have a similar precedent for such behavior with the
[.cfi_label](https://github.com/llvm/llvm-project/pull/97922)
instruction - so we add the `.loc_label THE_LABEL_NAME` instruction
which:
- Terminates the current line sequence in the line section
- Creates a new label with the specified label name in the `.debug_line`
section

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MCObjectStreamer: Remove an unneeded getBackendPtr test

All of `MCAsmBackend`, `MCCodeEmitter`, and `MCObjectWriter` must be
non-null.

MCAssembler: Move FileNames and CompilerVersion to MCObjectWriter

[MC][ELF] Emit instructions directly into fragment (#94950)

Avoid needless copying of instructions and fixups and directly emit into
the fragment small vectors.

This (optionally, second commit)

[MC][ELF] Emit instructions directly into fragment (#94950)

Avoid needless copying of instructions and fixups and directly emit into
the fragment small vectors.

This (optionally, second commit) also removes the single use of the
MCCompactEncodedInstFragment to simplify code.

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[MC] Remove nullable getCurrentSectionOnly use from AsmParser

We will implement getCurrentSectionOnly with `CurFrag->getParent()`,
which is non-null. Eliminate a nullable use.

[MC] Replace one nullable getCurrentSectionOnly with CurFrag

We will implement getCurrentSectionOnly with `CurFrag->getParent()`,
which is non-null. Eliminate a nullable use.

[MC] switchSectionNoPrint: update CurFrag

To ensure that CurFrag->Parent and SectionStack.back() are in sync.

[MC] Remove unneeded nullness check from getOrCreateDataFragment. NFC

`getCurrentFragment()` no longer returns null after
7423bf78eb53d81ce0c7b3a38e39a56341ca2a89.

[MC] Cache current fragment in MCStreamer

This eliminates indirection through `getCurrentSectionOnly()->curFragList()->Tail`.

[MC] Ensure subsections have a MCDataFragment

Similar to 21fac2d1d060b0f9b11a746718e58d4cd1ee97e5 for sections. This
makes it feasible to cache the current fragment in MCStreamer.

[MC,X86] emitInstruction: remove virtual function calls due to Intel JCC Erratum

https://reviews.llvm.org/D70157 (for Intel Jump Conditional Code
Erratum) introduced two virtual function calls in th

[MC,X86] emitInstruction: remove virtual function calls due to Intel JCC Erratum

https://reviews.llvm.org/D70157 (for Intel Jump Conditional Code
Erratum) introduced two virtual function calls in the generic
MCObjectStreamer::emitInstruction, which added some overhead.

This patch removes the virtual function overhead:

* Define `llvm::X86_MC::emitInstruction` that calls `emitInstruction{Begin,End}`.
* Define {X86ELFStreamer,X86WinCOFFStreamer}::emitInstruction to call `llvm::X86_MC::emitInstruction`

Pull Request: https://github.com/llvm/llvm-project/pull/96835

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[MC] Remove unused MCObjectStreamer::CurSubsectionIdx. NFC

[MC] Change Subsection parameters from const MCExpr * to uint32_t

Follow-up to 05ba5c0648ae5e80d5afce270495bf3b1eef9af4. uint32_t is
preferred over const MCExpr * in the section stack uses because i

[MC] Change Subsection parameters from const MCExpr * to uint32_t

Follow-up to 05ba5c0648ae5e80d5afce270495bf3b1eef9af4. uint32_t is
preferred over const MCExpr * in the section stack uses because it
should only be evaluated once. Change the paramter type to match.

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[MC] MCSectionSubPair: replace const MCExpr * with uint32_t

[MC] Remove remnant code related to pending labels

[MC] Remove pending labels

This commit removes the complexity introduced by pending labels in
https://reviews.llvm.org/D5915 by using a simpler approach. D5915 aimed
to ensure padding placement befo

[MC] Remove pending labels

This commit removes the complexity introduced by pending labels in
https://reviews.llvm.org/D5915 by using a simpler approach. D5915 aimed
to ensure padding placement before `.Ltmp0` for the following code, but
at the cost of expensive per-instruction `flushPendingLabels`.

```
// similar to llvm/test/MC/X86/AlignedBundling/labeloffset.s
.bundle_lock align_to_end
calll .L0$pb
.bundle_unlock
.L0$pb:
popl %eax
.Ltmp0: //// padding should be inserted before this label instead of after
addl $_GLOBAL_OFFSET_TABLE_+(.Ltmp0-.L0$pb), %eax
```

(D5915 was adjusted by https://reviews.llvm.org/D8072 and
https://reviews.llvm.org/D71368)

This patch achieves the same goal by setting the offset of the empty
MCDataFragment (`Prev`) in `layoutBundle`. This eliminates the need for
pending labels and simplifies the code.

llvm/test/MC/MachO/pending-labels.s (D71368): relocation symbols are
changed, but the result is still supported by linkers.

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[MC] emitLabelAtPos: change parameter to MCDataFragment &. NFC

emitLabelAtPos is only called by ARMELFStreamer with MCDataFragment.

[MC] Remove the Parent parameter from MCFragment ctor callers. NFC

[MC] Aligned bundling: remove special handling for RelaxAll

When both aligned bundling and RelaxAll are enabled, bundle padding is
directly written into fragments (https://reviews.llvm.org/D8072).
(

[MC] Aligned bundling: remove special handling for RelaxAll

When both aligned bundling and RelaxAll are enabled, bundle padding is
directly written into fragments (https://reviews.llvm.org/D8072).
(The original motivation was memory usage, which has been achieved from
different angles with recent assembler improvement).

The code presents challenges with the work to replace fragment
representation (e.g. #94950 #95077). This patch removes the special
handling. RelaxAll still works but the behavior seems slightly different
as revealed by 2 changed tests. However, most `-mc-relax-all` tests are
unchanged.

RelaxAll used to be the default for clang -O0. This mode has significant
code size drawbacks and newer Clang doesn't use it (#90013).

---

flushPendingLabels: The FOffset parameter can be removed: pending labels
will be assigned to the incoming fragment at offset 0.

Pull Request: https://github.com/llvm/llvm-project/pull/95188

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[MC] Add MCFragment allocation helpers

`allocFragment` might be changed to a placement new when the allocation
strategy changes.

`allocInitialFragment` is to deduplicate the following pattern
```

[MC] Add MCFragment allocation helpers

`allocFragment` might be changed to a placement new when the allocation
strategy changes.

`allocInitialFragment` is to deduplicate the following pattern
```
auto *F = new MCDataFragment();
Result->addFragment(*F);
F->setParent(Result);
```

Pull Request: https://github.com/llvm/llvm-project/pull/95197

show more ...

[MC] Restore setOffset(0) in emitLabel

For bolt/test/runtime/X86/exceptions-pic.test, llvm-bolt seems to call
emitLabel twice and the assert will fail. Work around it after
2cc4bc132cbcc76c5552cbc12

[MC] Restore setOffset(0) in emitLabel

For bolt/test/runtime/X86/exceptions-pic.test, llvm-bolt seems to call
emitLabel twice and the assert will fail. Work around it after
2cc4bc132cbcc76c5552cbc128830943ea596b3e

show more ...

MCFragment: Initialize Offset to 0

After 9d0754ada5dbbc0c009bcc2f7824488419cc5530 ("[MC] Relax fragments
eagerly") removes the assert of Offset, it is no longer useful to
initialize the member to -1

MCFragment: Initialize Offset to 0

After 9d0754ada5dbbc0c009bcc2f7824488419cc5530 ("[MC] Relax fragments
eagerly") removes the assert of Offset, it is no longer useful to
initialize the member to -1.

Now the symbol value estimate is more precise, which leads to slight
behavior change to layout-interdependency.s.

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[MC] Replace fragment ilist with singly-linked lists

Fragments are allocated with `operator new` and stored in an ilist with
Prev/Next/Parent pointers. A more efficient representation would be an
ar

[MC] Replace fragment ilist with singly-linked lists

Fragments are allocated with `operator new` and stored in an ilist with
Prev/Next/Parent pointers. A more efficient representation would be an
array of fragments without the overhead of Prev/Next pointers.

As the first step, replace ilist with singly-linked lists.

* `getPrevNode` uses have been eliminated by previous changes.
* The last use of the `Prev` pointer remains: for each subsection, there is an insertion point and
the current insertion point is stored at `CurInsertionPoint`.
* `HexagonAsmBackend::finishLayout` needs a backward iterator. Save all
fragments within `Frags`. Hexagon programs are usually small, and the
performance does not matter that much.

To eliminate `Prev`, change the subsection representation to
singly-linked lists for subsections and a pointer to the active
singly-linked list. The fragments from all subsections will be chained
together at layout time.

Since fragment lists are disconnected before layout time, we can remove
`MCFragment::SubsectionNumber` (https://reviews.llvm.org/D69411). The
current implementation of `AttemptToFoldSymbolOffsetDifference` requires
future improvement for robustness.

Pull Request: https://github.com/llvm/llvm-project/pull/95077

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[MC] Remove getFragmentList uses. NFC