Progress:

Changes

[Attributor] Generalize GEP construction

Make use of the getGEPIndicesForOffset() helper for creating GEPs.
This handles arrays as well, uses correct GEP index types and
reduces code duplication.

Differential Revision: https://reviews.llvm.org/D112263

[Loads] Use more powerful constant folding API

This follows up on D111023 by exporting the generic "load value
from constant at given offset as given type" and using it in the
store to load forwarding code. We now need to make sure that the
load size is smaller than the store size, previously this was
implicitly ensured by ConstantFoldLoadThroughBitcast().

Differential Revision: https://reviews.llvm.org/D112260

[libc++] Fix tests after aee49255074f

[llvm]Inclusive language: replace master with main

[NFC] This patch fixes a url in a testcase due to the renaming of the branch.

[lldb] Pass the target triple when determining the DWARF version

When targeting iOS, the default dwarf version is 2 and not 4. Currently,
the test suite does not pick up on that because it invokes the test
compiler without a target triple. This patch fixes that and now
correctly skips tests that have a dwarf version specified in a skipIf
decorator.

rdar://84530477

Differential revision: https://reviews.llvm.org/D112325

[TargetLowering] Simplify the interface of expandABS. NFC

Instead of returning a bool to indicate success and a separate
SDValue, return the SDValue and have the callers check if it is
null.

Reviewed By: RKSimon

Differential Revision: https://reviews.llvm.org/D112331

[DebugInfo][InstrRef] Add unit tests for transfer-function building

This patch adds some unit tests for the machine-location transfer-function
building parts of InstrRefBasedLDV: i.e., test that if we feed some MIR
into the transfer-function building code, does it create the correct
transfer function.

There are a number of minor defects that get corrected in the process:
 * The unit test was selecting the x86 (i.e. 32 bit) backend rather than
   x86_64's 64 bit backend,
 * COPY instructions weren't actually having their subregister values
   correctly represented in the transfer function. Subregisters were being
   defined by the COPY, rather than taking the value in the source register.
 * SP aliases were at risk of being clobbered, if an SP subregister was
   clobbered.

Differential Revision: https://reviews.llvm.org/D112006

[indvars] Use fact loop must exit to canonicalize to unsigned conditions

The logic in this patch is that if we find a comparison which would be unsigned except for when the loop is infinite, and we can prove that an infinite loop must be ill defined, we can still make the predicate unsigned.

The eventual goal (combined with a follow on patch) is to use the fact the loop exits to remove the zext (see tests) entirely.

A couple of points worth noting:
* We loose the ability to prove the loop unreachable by committing to the must exit interpretation. If instead, we later proved that rhs was definitely outside the range required for finiteness, we could have killed the loop entirely. (We don't currently implement this transform, but could in theory, do so.)
* simplifyAndExtend has a very limited list of users it walks. In particular, in the examples is stops at the zext and never visits the icmp. (Because we can't fold the zext to an addrec yet in SCEV.) Being willing to visit when we haven't simplified regresses multiple tests (seemingly because of less optimal results when computing trip counts).  D112170 explores fixing that, but - at least so far - appears to be too expensive compile time wise.

Differential Revision: https://reviews.llvm.org/D111836

[DAG] narrowExtractedVectorLoad - EXTRACT_SUBVECTOR indices are always constant

EXTRACT_SUBVECTOR indices are always constant, we don't need to check for ConstantSDNode, we should just use getConstantOperandVal which will assert for the constant.

[InstCombine] Various tests for truncating saturates and related patterns.

[flang] Fix bogus folding error for ISHFT(x, negative)

Negative shift counts are of course valid for ISHFT when
shifting to the right.  This patch decouples the folding of
ISHFT from that of SHIFTA/L/R and adds tests.

Differential Revision: https://reviews.llvm.org/D112244

[LegalizeTypes] Only expand CTLZ/CTTZ/CTPOP during type promotion if the new type is legal.

We might be promoting a large non-power of 2 type and the new type
may need to be split. Once we split it we may have a ctlz/cttz/ctpop
instruction for the split type.

I'm also concerned that we may create large shifts with shift amounts
that are too small.

[flang] Emit unformatted headers & footers even with RECL=

The runtime library was emitting unformatted record headers and
footers when an external unit had no fixed RECL=.  This is wrong
for sequential files, which should have headers & footers even
with RECL.  Change to omit headers & footers from unformatted
I/O only for direct access files.

Differential Revision: https://reviews.llvm.org/D112243

[DebugInfo][Instr] Track subregisters across stack spills/restores

Sometimes we generate code that writes to a subregister, then spills /
restores a super-register to the stack, for example:

    $eax = MOV32ri 0
    MOV64mr $rsp, 1, $noreg, 16, $noreg, $rax
    $rcx = MOV64rm $rsp, 1, $noreg, 8, $noreg

This patch takes a different approach: it adds another index to
MLocTracker that identifies a size/offset within a stack slot. A location
on the stack is then a pari of {FrameIndex, SlotNum}. Spilling and
restoring now involves pairing up the src/dest register numbers, and the
dest/src stack position to be transferred to/from. Location coverage
improves as a result, compile-time performance decreases, alas.

One limitation is that if a PHI occurs inside a stack slot:

    DBG_PHI %stack.0, 1

We don't know how large the resulting value is, and so might have
difficulty picking which value to use. DBG_PHI might need to be augmented
in the future with such a size.

Unit tests added ensure that spills and restores correctly transfer to
positions in the Location => Value map, and that different register classes
written to the stack will correctly clobber all other positions in the
stack slot.

Differential Revision: https://reviews.llvm.org/D112133

[flang] Enforce rest of semantic constraint C919

A reference to an allocatable or pointer component must be applied
to a scalar base object.  (This is the second part of constraint C919;
the first part is already checked.)

Differential Revision: https://reviews.llvm.org/D112241

[libunwind] Fix path to libunwind for per-target-runtime-dir builds

We recently introduced a from-scratch config to run the libunwind tests.
However, that config was always looking for libunwind in <install>/lib,
and never in <install>/<target>/lib, which is necessary for tests to
work when the per-target-runtime-dir configuration is enabled.

This commit fixes that. I believe this is what caused the CI failures we
saw after 5a8ad80b6fa5 and caused it to be reverted.

Differential Revision: https://reviews.llvm.org/D112322

[Dexter] Add DexFinishTest command to conditionally early-exit a test program

This patch adds a command, DexFinishTest, that allows a Dexter test to
be conditionally finished at a given breakpoint. This command has the
same set of arguments as DexLimitSteps, except that it does not allow a
line range (from_line, to_line), only a single line (on_line).

Reviewed By: Orlando

Differential Revision: https://reviews.llvm.org/D111988

[ConstantFolding] Drop misleading comment (NFC)

As pointed out by Philip, this part of the comment is misleading,
as it describes undef rather than poison behavior. Just mentioning
poison should be sufficient.

Support: Add Expected<T>::moveInto() to avoid extra names

Expected<T>::moveInto() takes as an out parameter any `OtherT&` that's
assignable from `T&&`. It moves any stored value before returning
takeError().

Since moveInto() consumes both the Error and the value, it's only
anticipated that we'd use call it on temporaries/rvalues, with naming
the Expected first likely to be an anti-pattern of sorts (either you
want to deal with both at the same time, or you don't). As such,
starting it out as `&&`-qualified... but it'd probably be fine to drop
that if there's a good use case for lvalues that appears.

There are two common patterns that moveInto() cleans up:
```
  // If the variable is new:
  Expected<std::unique_ptr<int>> ExpectedP = makePointer();
  if (!ExpectedP)
    return ExpectedP.takeError();
  std::unique_ptr<int> P = std::move(*ExpectedP);

  // If the target variable already exists:
  if (Expected<T> ExpectedP = makePointer())
    P = std::move(*ExpectedP);
  else
    return ExpectedP.takeError();
```
moveInto() takes less typing and avoids needing to name (or leak into
the scope) an extra variable.
```
  // If the variable is new:
  std::unique_ptr<int> P;
  if (Error E = makePointer().moveInto(P))
    return E;

  // If the target variable already exists:
  if (Error E = makePointer().moveInto(P))
    return E;
```

It also seems useful for unit tests, to log errors (but continue) when
there's an unexpected failure. E.g.:
```
  // Crash on error, or undefined in non-asserts builds.
  std::unique_ptr<MemoryBuffer> MB = cantFail(makeMemoryBuffer());

  // Avoid crashing on error without moveInto() :(.
  Expected<std::unique_ptr<MemoryBuffer>>
      ExpectedMB = makeMemoryBuffer();
  ASSERT_THAT_ERROR(ExpectedMB.takeError(), Succeeded());
  std::unique_ptr<MemoryBuffer> MB = std::move(ExpectedMB);

  // Avoid crashing on error with moveInto() :).
  std::unique_ptr<MemoryBuffer> MB;
  ASSERT_THAT_ERROR(makeMemoryBuffer().moveInto(MB), Succeeded());
```

Differential Revision: https://reviews.llvm.org/D112278