Allow passing LLVM downstream args to LLVMBuilder (#9491)

This allows the user to directly set LLVM backend options, e.g.
`-Xllvm... -unroll-count=0 -vectorize-loops=false -X.`. These parameters
are the ones you can set with `-mllvm` in Clang, i.e. the LLVM backend
parameters instead of Clang frontend parameters.

(I'm trying to get Slang's LLVM emitter to produce object code that I
can run on the Raspberry Pi Pico 2's Cortex M33 and need access to these
options for that purpose, but I think these will also be useful for
debugging the LLVM optimization passes later on.)

---------

Co-authored-by: Ellie Hermaszewska <ellieh@nvidia.com>

Author: juliusikkala

source/slang-llvm/slang-llvm-builder.cpp

@@ -4,6 +4,7 @@
 #include "slang-llvm-jit-shared-library.h"
 
 #include "llvm/AsmParser/Parser.h"
+#include "llvm/CodeGen/CommandFlags.h"
 #include "llvm/ExecutionEngine/Orc/LLJIT.h"
 #include "llvm/IR/BasicBlock.h"
 #include "llvm/IR/Constants.h"
@@ -39,6 +40,10 @@ namespace slang_llvm
 
 using namespace Slang;
 
+// This instance needs to exist so that we gain access to codegen flags in the
+// downstream arguments.
+static llvm::codegen::RegisterCodeGenFlags CGF;
+
 // There doesn't appear to be an LLVM output stream for writing into a memory
 // buffer. This implementation saves all written data directly into Slang's List
 // type.
@@ -502,6 +507,30 @@ LLVMBuilder::LLVMBuilder(LLVMBuilderOptions options, IArtifact** outErrorArtifac
 
     byteType = llvmBuilder->getInt8Ty();
 
+    // ParseCommandLineOptions below assumes that the first parameter is the
+    // name of the executable, but we do this cute trick to make it complain
+    // about parameters forwarded to LLVM instead:
+    std::vector<const char*> llvmArgs = {"-Xllvm"};
+    for (TerminatedCharSlice arg : options.llvmArguments)
+        llvmArgs.push_back(arg.data);
+
+    // Parse LLVM options. These can be used to adjust the behavior of various
+    // LLVM passes.
+    llvm::raw_string_ostream errorStream(error);
+    if (!llvm::cl::ParseCommandLineOptions(
+            llvmArgs.size(),
+            llvmArgs.data(),
+            "slangc",
+            &errorStream))
+    {
+        ArtifactDiagnostic diagnostic;
+        diagnostic.severity = ArtifactDiagnostic::Severity::Error;
+        diagnostic.stage = ArtifactDiagnostic::Stage::Link;
+        diagnostic.text = TerminatedCharSlice(error.c_str(), error.length());
+        *outErrorArtifact = createErrorArtifact(diagnostic);
+        return;
+    }
+
     if (options.debugLevel != SLANG_DEBUG_INFO_LEVEL_NONE)
     {
         llvmModule->addModuleFlag(
@@ -2244,7 +2273,7 @@ SlangResult LLVMBuilder::generateJITLibrary(IArtifact** outArtifact)
 
 } // namespace slang_llvm
 
-extern "C" SLANG_DLL_EXPORT SlangResult createLLVMBuilder_V1(
+extern "C" SLANG_DLL_EXPORT SlangResult createLLVMBuilder_V2(
     const SlangUUID& intfGuid,
     Slang::ILLVMBuilder** out,
     Slang::LLVMBuilderOptions options,

source/slang-llvm/slang-llvm-builder.h

@@ -70,6 +70,7 @@ struct LLVMBuilderOptions
     SlangFpDenormalMode fp32DenormalMode;
     SlangFpDenormalMode fp64DenormalMode;
     SlangFloatingPointMode fpMode;
+    Slice<TerminatedCharSlice> llvmArguments;
 };
 
 enum LLVMAttribute : uint32_t

source/slang/slang-emit-llvm.cpp

@@ -789,13 +789,13 @@ struct LLVMEmitter
             return SLANG_FAIL;
         }
 
-        using BuilderFuncV1 = SlangResult (*)(
+        using BuilderFuncV2 = SlangResult (*)(
             const SlangUUID& intfGuid,
             Slang::ILLVMBuilder** out,
             Slang::LLVMBuilderOptions options,
             Slang::IArtifact** outErrorArtifact);
 
-        auto builderFunc = (BuilderFuncV1)library->findFuncByName("createLLVMBuilder_V1");
+        auto builderFunc = (BuilderFuncV2)library->findFuncByName("createLLVMBuilder_V2");
         if (!builderFunc)
             return SLANG_FAIL;
 
@@ -828,6 +828,12 @@ struct LLVMEmitter
         builderOpt.fp64DenormalMode = (SlangFpDenormalMode)getOptions().getDenormalModeFp64();
         builderOpt.fpMode = (SlangFloatingPointMode)getOptions().getFloatingPointMode();
 
+        List<TerminatedCharSlice> llvmArguments;
+        List<String> downstreamArgs = getOptions().getDownstreamArgs("llvm");
+        for (const auto& arg : downstreamArgs)
+            llvmArguments.add(TerminatedCharSlice(arg.getBuffer()));
+        builderOpt.llvmArguments = Slice(llvmArguments.begin(), llvmArguments.getCount());
+
         ComPtr<IArtifact> errorArtifact;
         SLANG_RETURN_ON_FAIL(builderFunc(
             ILLVMBuilder::getTypeGuid(),