[DTLTO][LLD][COFF] Add support for Integrated Distributed ThinLTO (#148594)

This patch introduces support for Integrated Distributed ThinLTO (DTLTO)
in COFF LLD.

DTLTO enables the distribution of ThinLTO backend compilations via
external distribution systems, such as Incredibuild, during the
traditional link step: https://llvm.org/docs/DTLTO.html.

Note: Bitcode members of non-thin archives are not currently supported.
This will be addressed in a future change. This patch is sufficient to
allow for self-hosting an LLVM build with DTLTO if thin archives are
used.

Testing:
- LLD `lit` test coverage has been added, using a mock distributor to
avoid requiring Clang.
- Cross-project `lit` tests cover integration with Clang.

For the design discussion of the DTLTO feature, see:
https://github.com/llvm/llvm-project/pull/126654

3 files changed, 135 insertions, 1 deletions

diff --git a/cross-project-tests/dtlto/link-archive-thin.test b/cross-project-tests/dtlto/link-archive-thin.test
new file mode 100644
index 000000000000..fbd8fd67300c
--- /dev/null
+++ b/cross-project-tests/dtlto/link-archive-thin.test
@@ -0,0 +1,93 @@
+REQUIRES: lld-link
+
+## Test that a DTLTO link succeeds and outputs the expected set of files
+## correctly when thin archives are present.
+
+RUN: rm -rf %t && split-file %s %t && cd %t
+
+## Compile bitcode. -O2 is required for cross-module importing.
+RUN: %clang -O2 --target=x86_64-pc-windows-msvc -flto=thin -c \
+RUN:   foo.c bar.c dog.c cat.c start.c
+
+## Generate thin archives.
+RUN: lld-link /lib /llvmlibthin /out:foo.lib foo.o
+## Create this bitcode thin archive in a subdirectory to test the expansion of
+## the path to a bitcode file that is referenced using "..", e.g., in this case
+## "../bar.o".
+RUN: mkdir lib
+RUN: lld-link /lib /llvmlibthin /out:lib/bar.lib bar.o
+## Create this bitcode thin archive with an absolute path entry containing "..".
+RUN: lld-link /lib /llvmlibthin /out:dog.lib %t/lib/../dog.o
+RUN: lld-link /lib /llvmlibthin /out:cat.lib cat.o
+RUN: lld-link /lib /llvmlibthin /out:start.lib start.o
+
+## Link from a different directory to ensure that thin archive member paths are
+## resolved correctly relative to the archive locations.
+RUN: mkdir %t/out && cd %t/out
+RUN: lld-link /subsystem:console /machine:x64 /entry:start /out:my.exe  \
+RUN:   %t/foo.lib %t/lib/bar.lib ../start.lib %t/cat.lib \
+RUN:   /includeoptional:dog ../dog.lib \
+RUN:   -thinlto-distributor:%python \
+RUN:   -thinlto-distributor-arg:%llvm_src_root/utils/dtlto/local.py \
+RUN:   -thinlto-remote-compiler:%clang \
+RUN:   /lldsavetemps
+
+## Check that the required output files have been created.
+RUN: ls | FileCheck %s --check-prefix=OUTPUTS --implicit-check-not=cat
+
+## JSON jobs description.
+OUTPUTS-DAG: my.[[PID:[a-zA-Z0-9_]+]].dist-file.json
+
+## Individual summary index files.
+OUTPUTS-DAG: start.1.[[PID]].native.o.thinlto.bc{{$}}
+OUTPUTS-DAG:   dog.2.[[PID]].native.o.thinlto.bc{{$}}
+OUTPUTS-DAG:   foo.3.[[PID]].native.o.thinlto.bc{{$}}
+OUTPUTS-DAG:   bar.4.[[PID]].native.o.thinlto.bc{{$}}
+
+## Native output object files.
+OUTPUTS-DAG: start.1.[[PID]].native.o{{$}}
+OUTPUTS-DAG:   dog.2.[[PID]].native.o{{$}}
+OUTPUTS-DAG:   foo.3.[[PID]].native.o{{$}}
+OUTPUTS-DAG:   bar.4.[[PID]].native.o{{$}}
+
+
+## It is important that cross-module inlining occurs for this test to show that Clang can
+## successfully load the bitcode file dependencies recorded in the summary indices.
+## Explicitly check that the expected importing has occurred.
+
+RUN: llvm-dis start.1.*.native.o.thinlto.bc -o - | \
+RUN:   FileCheck %s --check-prefixes=FOO,BAR,START
+
+RUN: llvm-dis dog.2.*.native.o.thinlto.bc -o - | \
+RUN:   FileCheck %s --check-prefixes=FOO,BAR,DOG,START
+
+RUN: llvm-dis foo.3.*.native.o.thinlto.bc -o - | \
+RUN:   FileCheck %s --check-prefixes=FOO,BAR,START
+
+RUN: llvm-dis bar.4.*.native.o.thinlto.bc -o - | \
+RUN:   FileCheck %s --check-prefixes=FOO,BAR,START
+
+FOO-DAG:   foo.o
+BAR-DAG:   bar.o
+DOG-DAG:   dog.o
+START-DAG: start.o
+
+
+#--- foo.c
+extern int bar(int), start(int);
+__attribute__((retain)) int foo(int x) { return x + bar(x) + start(x); }
+
+#--- bar.c
+extern int foo(int), start(int);
+__attribute__((retain)) int bar(int x) { return x + foo(x) + start(x); }
+
+#--- dog.c
+extern int foo(int), bar(int), start(int);
+__attribute__((retain)) int dog(int x) { return x + foo(x) + bar(x) + start(x); }
+
+#--- cat.c
+__attribute__((retain)) void cat(int x) {}
+
+#--- start.c
+extern int foo(int), bar(int);
+__attribute__((retain)) int start(int x) { return x + foo(x) + bar(x); }
diff --git a/cross-project-tests/dtlto/link-dtlto.c b/cross-project-tests/dtlto/link-dtlto.c
new file mode 100644
index 000000000000..0ab4ec57f115
--- /dev/null
+++ b/cross-project-tests/dtlto/link-dtlto.c
@@ -0,0 +1,41 @@
+// REQUIRES: lld-link
+
+/// Simple test that DTLTO works with a single input bitcode file and that
+/// --save-temps can be applied to the remote compilation.
+
+// RUN: rm -rf %t && mkdir %t && cd %t
+
+// RUN: %clang --target=x86_64-pc-windows-msvc -c -flto=thin %s -o dtlto.obj
+
+// RUN: lld-link /subsystem:console /entry:_start dtlto.obj \
+// RUN:   -thinlto-distributor:%python \
+// RUN:   -thinlto-distributor-arg:%llvm_src_root/utils/dtlto/local.py \
+// RUN:   -thinlto-remote-compiler:%clang \
+// RUN:   -thinlto-remote-compiler-arg:--save-temps
+
+/// Check that the required output files have been created.
+// RUN: ls | sort | FileCheck %s
+
+/// No files are expected before.
+// CHECK-NOT: {{.}}
+
+/// Linked ELF.
+// CHECK: {{^}}dtlto.exe{{$}}
+
+/// Produced by the bitcode compilation.
+// CHECK-NEXT: {{^}}dtlto.obj{{$}}
+
+/// --save-temps output for the backend compilation.
+// CHECK-NEXT: {{^}}dtlto.s{{$}}
+// CHECK-NEXT: {{^}}dtlto.s.0.preopt.bc{{$}}
+// CHECK-NEXT: {{^}}dtlto.s.1.promote.bc{{$}}
+// CHECK-NEXT: {{^}}dtlto.s.2.internalize.bc{{$}}
+// CHECK-NEXT: {{^}}dtlto.s.3.import.bc{{$}}
+// CHECK-NEXT: {{^}}dtlto.s.4.opt.bc{{$}}
+// CHECK-NEXT: {{^}}dtlto.s.5.precodegen.bc{{$}}
+// CHECK-NEXT: {{^}}dtlto.s.resolution.txt{{$}}
+
+/// No files are expected after.
+// CHECK-NOT: {{.}}
+
+int _start() { return 0; }
diff --git a/cross-project-tests/lit.cfg.py b/cross-project-tests/lit.cfg.py
index b35c643ac898..31c93923ac9e 100644
--- a/cross-project-tests/lit.cfg.py
+++ b/cross-project-tests/lit.cfg.py
@@ -19,7 +19,7 @@ config.name = "cross-project-tests"
 config.test_format = lit.formats.ShTest(not llvm_config.use_lit_shell)
 
 # suffixes: A list of file extensions to treat as test files.
-config.suffixes = [".c", ".cl", ".cpp", ".m"]
+config.suffixes = [".c", ".cl", ".cpp", ".m", ".test"]
 
 # excludes: A list of directories to exclude from the testsuite. The 'Inputs'
 # subdirectories contain auxiliary inputs for various tests in their parent