The input LLVM module may define an array of cpu_mapping structs,
describing the mapping between fields in a specified struct (usually
CPUArchState) and TCG globals.

Create a map between offsets into the specified struct and TCG globals
(name, size, number of elements, stride) by iterating over the global
cpu_mapping array.  The name of this array is configurable via
the --tcg-global-mappings flag.

Signed-off-by: Anton Johansson <anjo@rev.ng>
---
 .../helper-to-tcg/include/PrepareForTcgPass.h |  7 ++-
 .../helper-to-tcg/include/TcgGlobalMap.h      | 31 +++++++++++++
 .../PrepareForTcgPass/PrepareForTcgPass.cpp   | 43 +++++++++++++++++++
 .../helper-to-tcg/pipeline/Pipeline.cpp       |  9 ++--
 4 files changed, 85 insertions(+), 5 deletions(-)
 create mode 100644 subprojects/helper-to-tcg/include/TcgGlobalMap.h

diff --git a/subprojects/helper-to-tcg/include/PrepareForTcgPass.h b/subprojects/helper-to-tcg/include/PrepareForTcgPass.h
index a41edb4c2e..a731c70b4b 100644
--- a/subprojects/helper-to-tcg/include/PrepareForTcgPass.h
+++ b/subprojects/helper-to-tcg/include/PrepareForTcgPass.h
@@ -17,11 +17,16 @@
 
 #pragma once
 
+#include "TcgGlobalMap.h"
 #include <llvm/IR/PassManager.h>
 
 class PrepareForTcgPass : public llvm::PassInfoMixin<PrepareForTcgPass> {
+    TcgGlobalMap &ResultTcgGlobalMap;
 public:
-    PrepareForTcgPass() {}
+    PrepareForTcgPass(TcgGlobalMap &ResultTcgGlobalMap)
+        : ResultTcgGlobalMap(ResultTcgGlobalMap)
+    {
+    }
     llvm::PreservedAnalyses run(llvm::Module &M,
                                 llvm::ModuleAnalysisManager &MAM);
 };
diff --git a/subprojects/helper-to-tcg/include/TcgGlobalMap.h b/subprojects/helper-to-tcg/include/TcgGlobalMap.h
new file mode 100644
index 0000000000..7186d805ba
--- /dev/null
+++ b/subprojects/helper-to-tcg/include/TcgGlobalMap.h
@@ -0,0 +1,31 @@
+#pragma once
+
+//
+//  Copyright(c) 2024 rev.ng Labs Srl. All Rights Reserved.
+//
+//  This program is free software; you can redistribute it and/or modify
+//  it under the terms of the GNU General Public License as published by
+//  the Free Software Foundation; either version 2 of the License, or
+//  (at your option) any later version.
+//
+//  This program is distributed in the hope that it will be useful,
+//  but WITHOUT ANY WARRANTY; without even the implied warranty of
+//  MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+//  GNU General Public License for more details.
+//
+//  You should have received a copy of the GNU General Public License
+//  along with this program; if not, see <http://www.gnu.org/licenses/>.
+//
+
+#include <llvm/ADT/StringRef.h>
+#include <llvm/ADT/DenseMap.h>
+#include <stdint.h>
+
+struct TcgGlobal {
+  llvm::StringRef Code;
+  uint64_t Size;
+  uint64_t NumElements;
+  uint64_t Stride;
+};
+
+using TcgGlobalMap = llvm::DenseMap<uint32_t, TcgGlobal>;
diff --git a/subprojects/helper-to-tcg/passes/PrepareForTcgPass/PrepareForTcgPass.cpp b/subprojects/helper-to-tcg/passes/PrepareForTcgPass/PrepareForTcgPass.cpp
index a2808eafed..a453aa8558 100644
--- a/subprojects/helper-to-tcg/passes/PrepareForTcgPass/PrepareForTcgPass.cpp
+++ b/subprojects/helper-to-tcg/passes/PrepareForTcgPass/PrepareForTcgPass.cpp
@@ -15,6 +15,7 @@
 //  along with this program; if not, see <http://www.gnu.org/licenses/>.
 //
 
+#include <CmdLineOptions.h>
 #include <PrepareForTcgPass.h>
 #include <llvm/ADT/SCCIterator.h>
 #include <llvm/IR/Function.h>
@@ -71,11 +72,53 @@ inline void demotePhis(Function &F)
     }
 }
 
+static void collectTcgGlobals(Module &M, TcgGlobalMap &ResultTcgGlobalMap)
+{
+    auto *Map = M.getGlobalVariable(TcgGlobalMappingsName);
+    if (!Map) {
+        return;
+    }
+
+    // In case the `tcg_global_mappings` array is empty,
+    // casting to `ConstantArray` will fail, even though it's a
+    // `[0 x %struct.cpu_tcg_mapping]`.
+    auto *MapElems = dyn_cast<ConstantArray>(Map->getOperand(0));
+    if (!MapElems) {
+        return;
+    }
+
+    for (auto Row : MapElems->operand_values()) {
+        auto *ConstRow = cast<ConstantStruct>(Row);
+
+        // Get code string
+        auto *CodePtr = ConstRow->getOperand(0);
+        auto CodeStr =
+            cast<ConstantDataArray>(
+                cast<Constant>(CodePtr->getOperand(0))->getOperand(0))
+                ->getAsString();
+        CodeStr = CodeStr.rtrim('\0');
+
+        // Get offset in cpu env
+        auto *Offset = cast<ConstantInt>(ConstRow->getOperand(3));
+        // Get size of variable in cpu env
+        auto *SizeInBytes = cast<ConstantInt>(ConstRow->getOperand(4));
+        unsigned SizeInBits = 8 * SizeInBytes->getLimitedValue();
+
+        auto *Stride = cast<ConstantInt>(ConstRow->getOperand(5));
+        auto *NumElements = cast<ConstantInt>(ConstRow->getOperand(6));
+
+        ResultTcgGlobalMap[Offset->getLimitedValue()] = {
+            CodeStr, SizeInBits, NumElements->getLimitedValue(),
+            Stride->getLimitedValue()};
+    }
+}
+
 PreservedAnalyses PrepareForTcgPass::run(Module &M, ModuleAnalysisManager &MAM)
 {
     removeFunctionsWithLoops(M, MAM);
     for (Function &F : M) {
         demotePhis(F);
     }
+    collectTcgGlobals(M, ResultTcgGlobalMap);
     return PreservedAnalyses::none();
 }
diff --git a/subprojects/helper-to-tcg/pipeline/Pipeline.cpp b/subprojects/helper-to-tcg/pipeline/Pipeline.cpp
index 7d03389439..a8df592af3 100644
--- a/subprojects/helper-to-tcg/pipeline/Pipeline.cpp
+++ b/subprojects/helper-to-tcg/pipeline/Pipeline.cpp
@@ -58,9 +58,9 @@ cl::opt<bool> TranslateAllHelpers(
 // Options for PrepareForTcgPass
 cl::opt<std::string> TcgGlobalMappingsName(
     "tcg-global-mappings",
-    cl::desc("<Name of global cpu_mappings[] used for mapping accesses"
-             "into a struct to TCG globals>"),
-    cl::Required, cl::cat(Cat));
+    cl::desc("Name of global cpu_mappings[] used for mapping accesses"
+             "into a struct to TCG globals"),
+    cl::init("mappings"), cl::cat(Cat));
 
 // Define a TargetTransformInfo (TTI) subclass, this allows for overriding
 // common per-llvm-target information expected by other LLVM passes, such
@@ -216,7 +216,8 @@ int main(int argc, char **argv)
     // easily to TCG.
     //
 
-    MPM.addPass(PrepareForTcgPass());
+    TcgGlobalMap TcgGlobals;
+    MPM.addPass(PrepareForTcgPass(TcgGlobals));
     MPM.addPass(VerifierPass());
     {
         FunctionPassManager FPM;
-- 
2.45.2