Add the ability to customise the pre-link pipeline

This adds `--newpm-passes <pipeline>`.

For consistency, this has the same semantics as LLVM's extant
`--lto-newpm-passes` switch.

Note that by imitating `--lto-newpm-passes` we inherit a quirk: if you
pass the empty string for the pipeline, you get the default pipeline,
and not an empty pipeline as one might expect.

This is because the switches are backed by a string, and the existence
of the switches is decided by checking for an empty string.

When the string is non-empty, it goes through a parsing function which
annoying also has no way to specify an empty pipeline.

Ideally the switches would be backed by a `optional<string>` (or the
parser would have a way to specify the empty pipeline), but: a) that
would be a separate change, and b) it should probably go upstream too.

For now, if you want an empty pipeline, I suggest you add a pass which
is a no-op on the IR, e.g. `verify`.

Author: vext01

clang/lib/CodeGen/BackendUtil.cpp

@@ -109,6 +109,10 @@ static cl::opt<bool> ClSanitizeOnOptimizerEarlyEP(
     cl::desc("Insert sanitizers on OptimizerEarlyEP."), cl::init(false));
 }
 
+static cl::opt<std::string> NewPMPasses("newpm-passes", cl::Optional,
+    cl::desc("Specify a custom pre-link pipeline. An empty string specifies the default pipeline."),
+    cl::init(""));
+
 namespace {
 
 // Default filename used for profile generation.
@@ -994,7 +998,13 @@ void EmitAssemblyHelper::RunOptimizationPipeline(
             MPM.addPass(InstrProfiling(*Options, false));
           });
 
-    if (CodeGenOpts.OptimizationLevel == 0) {
+    if (!NewPMPasses.empty()) {
+      MPM = PB.buildO0DefaultPipeline(Level, IsLTO || IsThinLTO);
+      if (auto Err = PB.parsePassPipeline(MPM, NewPMPasses)) {
+        report_fatal_error(Twine("unable to parse pass pipeline description '") +
+            NewPMPasses + "': " + toString(std::move(Err)));
+      }
+    } else if (CodeGenOpts.OptimizationLevel == 0) {
       MPM = PB.buildO0DefaultPipeline(Level, IsLTO || IsThinLTO);
     } else if (IsThinLTO) {
       MPM = PB.buildThinLTOPreLinkDefaultPipeline(Level);