[mlir] do not inject malloc/free in to-LLVM translation (#73224)

In the early days of MLIR-to-LLVM IR translation, it had to forcefully
inject declarations of `malloc` and `free` functions as then-standard
(now `memref`) dialect ops were unconditionally lowering to libc calls.
This is no longer the case. Even when they do lower to libc calls, the
signatures of those methods are injected at lowering since calls must
target declared functions in valid IR. Don't inject those declarations
anymore.

Author: ftynse

mlir/lib/Target/LLVMIR/ModuleTranslation.cpp

@@ -1369,14 +1369,6 @@ prepareLLVMModule(Operation *m, llvm::LLVMContext &llvmContext,
           m->getDiscardableAttr(LLVM::LLVMDialect::getTargetTripleAttrName()))
     llvmModule->setTargetTriple(cast<StringAttr>(targetTripleAttr).getValue());
 
-  // Inject declarations for `malloc` and `free` functions that can be used in
-  // memref allocation/deallocation coming from standard ops lowering.
-  llvm::IRBuilder<> builder(llvmContext);
-  llvmModule->getOrInsertFunction("malloc", builder.getInt8PtrTy(),
-                                  builder.getInt64Ty());
-  llvmModule->getOrInsertFunction("free", builder.getVoidTy(),
-                                  builder.getInt8PtrTy());
-
   return llvmModule;
 }
 

mlir/test/Target/LLVMIR/llvmir.mlir

@@ -179,7 +179,7 @@ llvm.mlir.global internal constant @sectionvar("teststring")  {section = ".mysec
 // CHECK: declare ptr @malloc(i64)
 llvm.func @malloc(i64) -> !llvm.ptr
 // CHECK: declare void @free(ptr)
-
+llvm.func @free(!llvm.ptr)
 
 //
 // Basic functionality: function and block conversion, function calls,