Fix DMD-style asm labels for LTO with template functions

When template functions containing DMD-style inline assembly are
instantiated in multiple modules and linked with LTO, duplicate
symbol errors can occur because the asm labels have identical names.

This fix assigns a unique ID to each function containing inline asm,
based on a deterministic hash of the template instantiation module
name. The ID is appended to label names, ensuring uniqueness across
compilation units.

Fixes: #4294

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

Author: Dev VM

dmd/declaration.h

@@ -565,6 +565,11 @@ class FuncDeclaration : public Declaration
     // Whether to emit instrumentation code if -fprofile-instr-generate is specified,
     // the value is set with pragma(LDC_profile_instr, true|false)
     bool emitInstrumentation;
+
+    // Unique ID for asm labels in this function, used to prevent duplicate
+    // symbol errors when LTO merges multiple instantiations of template functions.
+    // See: https://github.com/ldc-developers/ldc/issues/4294
+    unsigned asmLabelId;
 #endif
 
     VarDeclaration *vresult;            // result variable for out contracts

dmd/func.d

@@ -240,6 +240,11 @@ version (IN_LLVM)
     // Whether to emit instrumentation code if -fprofile-instr-generate is specified,
     // the value is set with pragma(LDC_profile_instr, true|false)
     bool emitInstrumentation = true;
+
+    // Unique ID for asm labels in this function, used to prevent duplicate
+    // symbol errors when LTO merges multiple instantiations of template functions.
+    // See: https://github.com/ldc-developers/ldc/issues/4294
+    uint asmLabelId = 0;
 }
 
     VarDeclaration vresult;             /// result variable for out contracts

gen/asm-x86.h

@@ -2416,7 +2416,9 @@ struct AsmProcessor {
   void addLabel(const char *id) {
     // We need to delay emitting the actual function name, see
     // replace_func_name in asmstmt.cpp for details.
-    printLabelName(insnTemplate, "<<func>>", id);
+    // Pass the function's asmLabelId to make labels unique across template
+    // instantiations when LTO merges them.
+    printLabelName(insnTemplate, "<<func>>", id, sc->func->asmLabelId);
   }
 
   /* Determines whether the operand is a register, memory reference

gen/asmstmt.cpp

@@ -11,8 +11,10 @@
 #include "dmd/dsymbol.h"
 #include "dmd/errors.h"
 #include "dmd/ldcbindings.h"
+#include "dmd/module.h"
 #include "dmd/scope.h"
 #include "dmd/statement.h"
+#include "dmd/template.h"
 #include "gen/dvalue.h"
 #include "gen/functions.h"
 #include "gen/irstate.h"
@@ -97,6 +99,12 @@ static void replace_func_name(IRState *p, std::string &insnt) {
   }
 }
 
+// Global counter for generating unique asm label IDs per function.
+// This prevents duplicate symbol errors when LTO merges multiple
+// instantiations of template functions with inline asm.
+// See: https://github.com/ldc-developers/ldc/issues/4294
+static unsigned nextAsmLabelId = 1;
+
 Statement *asmSemantic(AsmStatement *s, Scope *sc) {
   if (!s->tokens) {
     return nullptr;
@@ -106,10 +114,50 @@ Statement *asmSemantic(AsmStatement *s, Scope *sc) {
   if (s->tokens->value == TOK::string_ ||
       s->tokens->value == TOK::leftParenthesis) {
     auto gas = createGccAsmStatement(s->loc, s->tokens);
-    return gccAsmSemantic(gas, sc);
+    return dmd::gccAsmSemantic(gas, sc);
   }
 
   // this is DMD-style asm
+
+  // Assign a unique asm label ID to this function if it doesn't have one yet.
+  // This ID is used to make labels unique across different compilation units
+  // when LTO merges template instantiations.
+  //
+  // For template functions, we use the instantiation module's name to create
+  // a deterministic hash that differs across compilation units. This ensures:
+  // 1. Same compilation → same hash → reproducible builds
+  // 2. Different compilation units → different hash → no LTO conflicts
+  if (sc->func->asmLabelId == 0) {
+    unsigned moduleHash = 0;
+
+    // For template instantiations, use the instantiation module's name
+    // This is the module that caused this template to be instantiated,
+    // which differs between compilation units even for the same template.
+    if (auto ti = sc->func->isInstantiated()) {
+      if (ti->minst && ti->minst->ident) {
+        const char* modName = ti->minst->ident->toChars();
+        for (const char* p = modName; *p; ++p) {
+          moduleHash = moduleHash * 31 + static_cast<unsigned char>(*p);
+        }
+      }
+    }
+
+    // If not a template or no instantiation module, use the current module
+    if (moduleHash == 0 && sc->_module && sc->_module->ident) {
+      const char* modName = sc->_module->ident->toChars();
+      for (const char* p = modName; *p; ++p) {
+        moduleHash = moduleHash * 31 + static_cast<unsigned char>(*p);
+      }
+    }
+
+    // Combine module hash with counter for uniqueness within a compilation
+    // Use golden ratio constant for better bit mixing
+    sc->func->asmLabelId = (moduleHash * 2654435761u) ^ (nextAsmLabelId++ * 31);
+    // Ensure non-zero
+    if (sc->func->asmLabelId == 0) {
+      sc->func->asmLabelId = 1;
+    }
+  }
   sc->func->hasInlineAsm(true);
 
   const auto caseSensitive = s->caseSensitive();
@@ -513,7 +561,7 @@ void CompoundAsmStatement_toIR(CompoundAsmStatement *stmt, IRState *p) {
       static size_t uniqueLabelsId = 0;
       std::string suffix = "_llvm_asm_end";
       suffix += std::to_string(uniqueLabelsId++);
-      printLabelName(asmGotoEndLabel, fdmangle, suffix.c_str());
+      printLabelName(asmGotoEndLabel, fdmangle, suffix.c_str(), fd->asmLabelId);
     }
 
     // initialize the setter statement we're going to build
@@ -550,7 +598,7 @@ void CompoundAsmStatement_toIR(CompoundAsmStatement *stmt, IRState *p) {
       IF_LOG Logger::println(
           "statement '%s' references outer label '%s': creating forwarder",
           a->code.c_str(), ident->toChars());
-      printLabelName(code, fdmangle, ident->toChars());
+      printLabelName(code, fdmangle, ident->toChars(), fd->asmLabelId);
       code << ":\n\t";
       code << "movl $<<in" << n_goto << ">>, $<<out0>>\n";
       // FIXME: Store the value -> label mapping somewhere, so it can be

gen/llvmhelpers.cpp

@@ -48,6 +48,7 @@
 #include <llvm/IR/Constant.h>
 #include <llvm/Analysis/ConstantFolding.h>
 #include <stack>
+#include <unordered_map>
 
 using namespace dmd;
 
@@ -1369,11 +1370,18 @@ bool isLLVMUnsigned(Type *t) {
 ////////////////////////////////////////////////////////////////////////////////
 
 void printLabelName(std::ostream &target, const char *func_mangle,
-                    const char *label_name) {
+                    const char *label_name, unsigned asmLabelId) {
   // note: quotes needed for Unicode
   target << '"'
          << gTargetMachine->getMCAsmInfo()->getPrivateGlobalPrefix().str()
-         << func_mangle << "_" << label_name << '"';
+         << func_mangle << "_" << label_name;
+  // Append unique ID to prevent duplicate symbol errors when LTO merges
+  // multiple instantiations of template functions with inline asm.
+  // See: https://github.com/ldc-developers/ldc/issues/4294
+  if (asmLabelId > 0) {
+    target << "_" << asmLabelId;
+  }
+  target << '"';
 }
 
 ////////////////////////////////////////////////////////////////////////////////

gen/llvmhelpers.h

@@ -210,7 +210,7 @@ DValue *DtoCallFunction(Loc loc, Type *resulttype, DValue *fnval,
 Type *stripModifiers(Type *type, bool transitive = false);
 
 void printLabelName(std::ostream &target, const char *func_mangle,
-                    const char *label_name);
+                    const char *label_name, unsigned asmLabelId = 0);
 
 void AppendFunctionToLLVMGlobalCtorsDtors(llvm::Function *func,
                                           const uint32_t priority,

gen/naked.cpp

@@ -129,7 +129,7 @@ class ToNakedIRVisitor : public Visitor {
     LOG_SCOPE;
 
     printLabelName(irs->nakedAsm, mangleExact(irs->func()->decl),
-                   stmt->ident->toChars());
+                   stmt->ident->toChars(), irs->func()->decl->asmLabelId);
     irs->nakedAsm << ":";
 
     if (stmt->statement) {

gen/statements.cpp

@@ -1548,7 +1548,7 @@ class ToIRVisitor : public Visitor {
       auto a = new IRAsmStmt;
       std::stringstream label;
       printLabelName(label, mangleExact(irs->func()->decl),
-                     stmt->ident->toChars());
+                     stmt->ident->toChars(), irs->func()->decl->asmLabelId);
       label << ":";
       a->code = label.str();
       irs->asmBlock->s.push_back(a);

tests/codegen/asm_labels.d

@@ -8,9 +8,9 @@ void foo(int a)
 {
     asm
     {
-        // CHECK: jmp .L_D10asm_labels3fooFiZv_label
+        // CHECK: jmp .L_D10asm_labels3fooFiZv_label{{(_[0-9]+)?}}
         jmp label;
-        // CHECK-NEXT: .L_D10asm_labels3fooFiZv_label:
+        // CHECK-NEXT: .L_D10asm_labels3fooFiZv_label{{(_[0-9]+)?}}:
     label:
         ret;
     }
@@ -21,9 +21,9 @@ void foo(uint a)
 {
     asm
     {
-        // CHECK: jmp .L_D10asm_labels3fooFkZv_label
+        // CHECK: jmp .L_D10asm_labels3fooFkZv_label{{(_[0-9]+)?}}
         jmp label;
-        // CHECK-NEXT: .L_D10asm_labels3fooFkZv_label:
+        // CHECK-NEXT: .L_D10asm_labels3fooFkZv_label{{(_[0-9]+)?}}:
     label:
         ret;
     }

tests/linking/asm_labels_lto.d

@@ -0,0 +1,36 @@
+// Tests that DMD-style asm labels in naked template functions get unique IDs
+// to prevent "symbol already defined" errors during LTO linking.
+//
+// Without the fix, LTO linking fails with:
+//   symbol '.L_D...nakedAsmTemplate..._L1' is already defined
+//
+// See: https://github.com/ldc-developers/ldc/issues/4294
+
+// REQUIRES: LTO
+// REQUIRES: atleast_llvm1400
+// REQUIRES: target_X86
+
+// Compile two modules that each instantiate the same naked asm template.
+// RUN: %ldc -flto=full -c -I%S %S/inputs/asm_lto_user.d -of=%t_user%obj
+// RUN: %ldc -flto=full -c -I%S %s -of=%t_main%obj
+
+// Link with LTO - this fails without the fix due to duplicate asm labels.
+// RUN: %ldc -flto=full %t_main%obj %t_user%obj -of=%t%exe
+
+// Verify the executable runs correctly.
+// RUN: %t%exe
+
+module asm_labels_lto;
+
+import inputs.asm_lto_template;
+import inputs.asm_lto_user;
+
+int main() {
+    // Both modules instantiate nakedAsmTemplate!1
+    // Without unique label IDs, LTO linking fails with "symbol already defined"
+    uint a = nakedAsmTemplate!1();  // From this module's instantiation
+    uint b = useTemplate();          // From asm_lto_user's instantiation
+
+    // Both should return the same value (>= 100)
+    return (a == b && a >= 100) ? 0 : 1;
+}