This is an LLD extension to MSVC link.exe command line. MSVC linker
does not write symbol tables for executables. We do unless no /debug
option is given.

There's a situation that we want to enable debug info but don't want
to emit the symbol table. One example is when we are comparing output
file size. With this patch, you can tell the linker to not create
a symbol table by just specifying /nosymtab.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@248225 91177308-0d34-0410-b5e6-96231b3b80d8

Conceptually, garbage collection is not part of Writer,
so move the function out of the file.

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This patch saves 4 ms out of 5 ms. Very small improvement,
but maybe better than nothing.

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Self-hosting took 801 ms on my machine. Of which this function took
69 ms. Now it takes 37 ms. That is about 4% overall performance
improvement.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@248052 91177308-0d34-0410-b5e6-96231b3b80d8

Basically the concept of "liveness" is for sections (or chunks in LLD
terminology) and not for symbols. Symbols are always available or live,
or otherwise it indicates a link failure.

Previously, we had isLive() and markLive() methods for DefinedSymbol.
They are confusing methods. What they actually did is to act as a proxy
to backing section chunks. We can simplify eliminate these methods
and call section chunk's methods directly.

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Only live symbols are written to the symbol table. Because isLive()
returned false if dead-stripping was disabled entirely, only
non-COMDAT sections were written to the symbol table. This patch fixes
the issue.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@247856 91177308-0d34-0410-b5e6-96231b3b80d8

Now that ICF is complete, we can rename this option so that
the driver accepts the MSVC-compatible command line option.

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This patch fixes a subtle incompatibility with MSVC linker.
MSVC linker preserves the original spelling of a DLL in the
import descriptor table. LLD previously converted all
characters to lowercase. Usually this difference is benign,
but if a program explicitly checks for DLL file names, the
program could fail.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@246620 91177308-0d34-0410-b5e6-96231b3b80d8

There are some DLLs whose initializers depends on other DLLs'
initializers. The initialization order matters for them.

MSVC linker uses the order of the libraries from the command line.
LLD used ASCII-betical order. So they were incompatible.
This patch makes LLD compatible with MSVC.

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A short import library has up to two symbols, so we don't have
to do a for-loop and type dispatch in createImportTables.

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This is more convenient than the offset from the start of the file as we
don't have to worry about it changing when we move the output section.

This is a port of r245008 from ELF.

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Sections must start at page boundaries in memory, but they
can be aligned to sector boundaries (512-bytes) on disk.
We aligned them to 4096-byte boundaries even on disk, so we
wasted disk space a bit.

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Handling the symbol this way is consistent with other symbols, such as
_tls_used. NFC.

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This has a few advantages

* Less C++ code (about 300 lines less).
* Less machine code (about 14 KB of text on a linux x86_64 build).
* It is more debugger friendly. Just set a breakpoint on the exit function and
  you get the complete lld stack trace of when the error was found.
* It is a more robust API. The errors are handled early and we don't get a
  std::error_code hot potato being passed around.
* In most cases the error function in a better position to print diagnostics
  (it has more context).

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On ARM, exception handler entries in .pdata section are 8 byte long.

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Various parameters are passed implicitly using Config global variable
already. Output file path is no different from others, so there was no
special reason to handle that differnetly.

This patch changes the signature of writeResult(SymbolTable *, StringRef)
to writeResult(SymbolTable *).

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I don't remember why I thought that only functions are subject
of garbage collection, but the comment here said so, which is
not correct. Moreover, the code just below the comment does not
do what the comment says -- it handles non-COMDAT, non-function
sections as GC root. As a result, it just handles non-COMDAT
sections as GC root.

This patch cleans that up by removing SectionChunk::isRoot and
use isCOMDAT instead.

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We want to convince the NT loader not to map these sections into memory.
A good first step is to move them to the end of the executable.

Differential Revision: http://reviews.llvm.org/D11655

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Windows ARM is the thumb ARM environment, and pointers to thumb code
needs to have its LSB set. When we apply relocations, we need to
adjust the LSB if it points to an executable section.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@243560 91177308-0d34-0410-b5e6-96231b3b80d8

SECREL should sets the 32-bit offset of the target from the beginning
of *target's* output section. Previously, the offset from the beginning
of source's output section was used instead.

SECTION means the target section's index, and not the source section's
index. This patch fixes that issue too.

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Previously, we ignore /merge option if /debug is specified
because I thought that was MSVC linker did. This was wrong.
/merge shouldn't be ignored even in debug mode.

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Previously no __imp_ symbols nor dllimport thunk functions were
written to a symbol table.

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On x64 and x86, we use only one base relocation type, so we handled
base relocations just as a list of RVAs. That doesn't work well for
ARM becuase we have to handle two types of base relocations on ARM.
This patch changes the type of base relocation from uint32_t to
{reltype, uint32_t} to make it easy to port this code to ARM.

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An object file compatible with Safe SEH contains a .sxdata section.
The section contains a list of symbol table indices, each of which
is an exception handler function. A safe SEH-enabled executable
contains a list of exception handler RVAs. So, what the linker has
to do to support Safe SEH is basically to read the .sxdata section,
interpret the contents as a list of symbol indices, unique-fy and
sort their RVAs, and then emit that list to .rdata. This patch
implements that feature.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@243182 91177308-0d34-0410-b5e6-96231b3b80d8

__ImageBase is a special symbol whose value is the image base address.
Previously, we handled __ImageBase symbol as an absolute symbol.

Absolute symbols point to specific locations in memory and the locations
never change even if an image is base-relocated. That means that we
don't have base relocation entries for absolute symbols.

This is not a case for __ImageBase. If an image is base-relocated, its
base address changes, and __ImageBase needs to be shifted as well.
So we have to have base relocations for __ImageBase. That means that
__ImageBase is not really an absolute symbol but a different kind of
symbol.

In this patch, I introduced a new type of symbol -- DefinedRelative.
DefinedRelative is similar to DefinedAbsolute, but it has not a VA but RVA
and is a subject of base relocation. Currently only __ImageBase is of
the new symbol type.

git-svn-id: https://llvm.org/svn/llvm-project/lld/trunk@243176 91177308-0d34-0410-b5e6-96231b3b80d8

Load Configuration field points to a structure containing information
for SEH. That data strucutre is not created by the linker but provided
by an external file. What we have to do is just to set __load_config_used
address to the header.

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If /delayload option is given, we have to resolve __delayLoadHelper2
since the function is the dynamic loader to delay-load DLLs.
The function name is mangled in x86 as ___delayLoadHelper2@8.

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