[PATCH] Add ObjC ARC module flag and use it to control arc-contract pass

[Duncan P. N. Exon Smith]

> On 2015 Mar 16, at 10:34, Steven Wu <stevenwu at apple.com> wrote:
> 
> After a detailed looked at ObjCContract pass, it already has a module level check to decide whether the pass should be run or not. It checks the presence of ARC function at module level. The overhead of this check is very low and it doesn’t hurt to run it all the time. So there are three options now:
> 1. Function attribute: the advantage is that we can run ObjCContract pass only on function instead of the entire module. The disadvantage is that it introduces complication in the bicode and we have to run upgrade pass for old bitcode by checking all the presence of ARC function.
> 2. Module flag: it is really not helpful to create a module flag after I find there is almost no overhead to run ObjCARCContract pass on a module without Objc-ARC. It is also tricky to get the consistence behavior after the upgrade of bitcode by only checking the module flag.
> 3. Always run the pass to match the behavior in LTO: this seems the simplest solution. I also run full LNT test-suite to prove that there are no overhead to run the pass on the code without ObjC-ARC. I force ObjCARCContract pass to run at -O0 on the code and there are no observable overhead even at -O0. Results attached.
> 
> Steven
> <no-objc-arc-O0.json><objc-arc-O0.json>

(Did you mean to attach a patch to this email?)

#3 makes sense to me given how cheap the checks are.  I hadn't
realized they were on the module.

> 
> 
>> On Mar 3, 2015, at 1:20 PM, Duncan P. N. Exon Smith <dexonsmith at apple.com> wrote:
>> 
>> 
>>> On 2015-Mar-02, at 14:56, Steven Wu <stevenwu at apple.com> wrote:
>>> 
>>> Fix the code according to Duncan's feedback. Attempt to add a testcase.
>>> Not sure if there is better to check if the pass is run other than using -debug-pass=Arguments.
>> 
>> Steven and I talked offline.  All the passes that depend on this flag
>> are function passes.  Since it's possible to switch on a per-function
>> basis, this should be a function attribute (not a module flag).
>> 
>>> http://reviews.llvm.org/D7799
>>> 
>>> Files:
>>> lib/CodeGen/BackendUtil.cpp
>>> lib/CodeGen/CGObjCMac.cpp
>>> test/CodeGen/arc-passes.m
>>> 
>>> Index: lib/CodeGen/BackendUtil.cpp
>>> ===================================================================
>>> --- lib/CodeGen/BackendUtil.cpp
>>> +++ lib/CodeGen/BackendUtil.cpp
>>> @@ -569,7 +569,7 @@
>>> // Add ObjC ARC final-cleanup optimizations. This is done as part of the
>>> // "codegen" passes so that it isn't run multiple times when there is
>>> // inlining happening.
>>> -  if (LangOpts.ObjCAutoRefCount &&
>>> +  if (TheModule->getModuleFlag("Objective-C ARC") &&
>>>     CodeGenOpts.OptimizationLevel > 0)
>>>   PM->add(createObjCARCContractPass());
>>> 
>>> Index: lib/CodeGen/CGObjCMac.cpp
>>> ===================================================================
>>> --- lib/CodeGen/CGObjCMac.cpp
>>> +++ lib/CodeGen/CGObjCMac.cpp
>>> @@ -4284,6 +4284,12 @@
>>>   }
>>> }
>>> 
>>> +  // Add ObjCAutoRefCount as a module flag
>>> +  if (CGM.getLangOpts().ObjCAutoRefCount) {
>>> +    Mod.addModuleFlag(llvm::Module::Error,
>>> +                      "Objective-C ARC", 1u);
>>> +  }
>>> +
>>> // Indicate whether we're compiling this to run on a simulator.
>>> const llvm::Triple &Triple = CGM.getTarget().getTriple();
>>> if (Triple.isiOS() &&
>>> Index: test/CodeGen/arc-passes.m
>>> ===================================================================
>>> --- /dev/null
>>> +++ test/CodeGen/arc-passes.m
>>> @@ -0,0 +1,12 @@
>>> +// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -fobjc-arc -emit-llvm %s -o - | FileCheck %s
>>> +// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -emit-llvm %s -o - | FileCheck %s -check-prefix=NO-ARC
>>> +// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -fobjc-arc -emit-obj -O3 %s -o /dev/null -mllvm -debug-pass=Arguments 2>&1 | FileCheck %s -check-prefix=ARC-PASS
>>> +// RUN: %clang_cc1 -triple x86_64-apple-darwin10 -emit-obj -O3 %s -o /dev/null -mllvm -debug-pass=Arguments 2>&1 | FileCheck %s -check-prefix=NO-ARC-PASS
>>> +
>>> +// CHECK: !{i32 1, !"Objective-C ARC", i32 1}
>>> +// NO-ARC-NOT: !{i32 1, !"Objective-C ARC", i32 1}
>>> +// ARC-PASS: -objc-arc-contract
>>> +// NO-ARC-PASS-NOT: -objc-arc-contract
>>> +
>>> +void test() {
>>> +}
>>> 
>>> EMAIL PREFERENCES
>>> http://reviews.llvm.org/settings/panel/emailpreferences/
>>> <D7799.21048.patch>
>> 
>