[compiler] Remove the RemoveFencesPass

This pass was originally introduced back in LLVM 7 to account for the
fact that some of our targets couldn't handle the newly added `fence`
instruction. Removing them bypassed the issue.

While it is not semantically correct to remove fence instructions from
LLVM IR in general, we were relying on the fact that the memory model is
under-specified in OpenCL 1.2 and so could assume that fences were
redundant.

The ENORMOUS WARNING above this scheduling of this pass should be reason
enough not to run it. It has not been necessary for several LLVM
versions - it is claimed it is only in the host pipeline to ensure it's
tested. This is not a good enough reason to keep a dodgy unnecessary
pass.

There aren't enough good reasons to keep it around unused in the
codebase (the pass is trivial to implement by a downstream target if
they did require it) so this commit removes the pass.

doc/modules/compiler/utils.rst

@@ -1139,13 +1139,6 @@ Removing this information is useful for debugging since the backend is less
 likely to optimize away variables in the stack no longer used, as a result this
 pass should only be run on debug builds of the module.
 
-RemoveFencesPass
-----------------
-
-Removing memory fences can result in invalid code or incorrect behaviour in
-general. This pass is a workaround for backends that do not yet support memory
-fences.
-
 RemoveExceptionsPass
 --------------------
 

modules/compiler/source/base/source/base_module_pass_machinery.cpp

@@ -49,7 +49,6 @@
 #include <compiler/utils/prepare_barriers_pass.h>
 #include <compiler/utils/reduce_to_function_pass.h>
 #include <compiler/utils/remove_exceptions_pass.h>
-#include <compiler/utils/remove_fences_pass.h>
 #include <compiler/utils/remove_lifetime_intrinsics_pass.h>
 #include <compiler/utils/rename_builtins_pass.h>
 #include <compiler/utils/replace_address_space_qualifier_functions_pass.h>

modules/compiler/source/base/source/base_module_pass_registry.def

@@ -177,7 +177,6 @@ FUNCTION_PASS("software-div", compiler::SoftwareDivisionPass())
 FUNCTION_PASS("replace-addrspace-fns", compiler::utils::ReplaceAddressSpaceQualifierFunctionsPass())
 FUNCTION_PASS("remove-lifetime", compiler::utils::RemoveLifetimeIntrinsicsPass())
 FUNCTION_PASS("remove-exceptions", compiler::utils::RemoveExceptionsPass())
-FUNCTION_PASS("remove-fences", compiler::utils::RemoveFencesPass())
 FUNCTION_PASS("replace-mem-intrins", compiler::utils::ReplaceMemIntrinsicsPass())
 FUNCTION_PASS("print<generic-metadata>", compiler::utils::GenericMetadataPrinterPass(llvm::dbgs()))
 FUNCTION_PASS("print<vectorize-metadata>", compiler::utils::VectorizeMetadataPrinterPass(llvm::dbgs()))

modules/compiler/targets/host/source/HostPassMachinery.cpp

@@ -32,7 +32,6 @@
 #include <compiler/utils/metadata_analysis.h>
 #include <compiler/utils/pipeline_parse_helpers.h>
 #include <compiler/utils/remove_exceptions_pass.h>
-#include <compiler/utils/remove_fences_pass.h>
 #include <compiler/utils/remove_lifetime_intrinsics_pass.h>
 #include <compiler/utils/replace_address_space_qualifier_functions_pass.h>
 #include <compiler/utils/replace_local_module_scope_variables_pass.h>
@@ -311,20 +310,6 @@ llvm::ModulePassManager HostPassMachinery::getKernelFinalizationPasses(
         compiler::utils::RemoveLifetimeIntrinsicsPass()));
   }
 
-  // ENORMOUS WARNING:
-  // Removing memory fences can result in invalid code or incorrect behaviour in
-  // general. This pass is a workaround for backends that do not yet support
-  // memory fences.  This is not required for any of the LLVM backends used by
-  // host, but the pass is used here to ensure that it is tested.
-  // The memory model on OpenCL 1.2 is so underspecified that we can get away
-  // with removing fences. In OpenCL 3.0 the memory model is better defined, and
-  // just removing fences could result in incorrect behavior for valid 3.0
-  // OpenCL applications.
-  if (options.standard != compiler::Standard::OpenCLC30) {
-    PM.addPass(llvm::createModuleToFunctionPassAdaptor(
-        compiler::utils::RemoveFencesPass()));
-  }
-
   PM.addPass(compiler::utils::ComputeLocalMemoryUsagePass());
 
   PM.addPass(compiler::utils::AddMetadataPass<

modules/compiler/utils/CMakeLists.txt

@@ -52,7 +52,6 @@ add_ca_library(compiler-utils STATIC
   ${CMAKE_CURRENT_SOURCE_DIR}/include/compiler/utils/prepare_barriers_pass.h
   ${CMAKE_CURRENT_SOURCE_DIR}/include/compiler/utils/reduce_to_function_pass.h
   ${CMAKE_CURRENT_SOURCE_DIR}/include/compiler/utils/remove_exceptions_pass.h
-  ${CMAKE_CURRENT_SOURCE_DIR}/include/compiler/utils/remove_fences_pass.h
   ${CMAKE_CURRENT_SOURCE_DIR}/include/compiler/utils/remove_lifetime_intrinsics_pass.h
   ${CMAKE_CURRENT_SOURCE_DIR}/include/compiler/utils/rename_builtins_pass.h
   ${CMAKE_CURRENT_SOURCE_DIR}/include/compiler/utils/replace_address_space_qualifier_functions_pass.h
@@ -104,7 +103,6 @@ add_ca_library(compiler-utils STATIC
   ${CMAKE_CURRENT_SOURCE_DIR}/source/prepare_barriers_pass.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/source/reduce_to_function_pass.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/source/remove_exceptions_pass.cpp
-  ${CMAKE_CURRENT_SOURCE_DIR}/source/remove_fences_pass.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/source/remove_lifetime_intrinsics_pass.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/source/rename_builtins_pass.cpp
   ${CMAKE_CURRENT_SOURCE_DIR}/source/replace_address_space_qualifier_functions_pass.cpp