[llvm] r215600 - [AArch64, fast-isel] Fall back to SelectionDAG to select tail calls.

Fri Aug 15 06:23:26 PDT 2014

Sounds good to me, Bob.

> We should fix clang to switch over to using musttail attributes for
> those cases (<rdar://problem/18024244>). Once that is done, we can change
> fast-isel back to ignore tail attributes (<rdar://problem/18024310>).
>
>> On Aug 13, 2014, at 11:25 PM, Akira Hatanaka <ahatanaka at apple.com>
>> wrote:
>>
>> It looks like clang marks calls as tail only when the called function
>> is objc_autoreleaseReturnValue or objc_retainAutoreleaseReturnValue.
>> Also a call is marked as musttail in
>> CodeGenFunction::EmitMustTailThunk. Assuming you arent worried about
>> those cases, I guess narrowing down the scope of functions that are
>> emitted as tail calls in AArch64s fast-isel doesnt make much
>> difference?
>>
>> On Aug 13, 2014, at 6:03 PM, Chad Rosier <mcrosier at codeaurora.org>
>> wrote:
>>
>>> Seems musttail was landed back in April, but the Phabricator revision
>>> was
>>> never closed.  Sounds like a good solution, but again no rush.
>>>
>>>> Akira,
>>>> Has the musttail attribute landed?  Looks like Reid was working on
>>>> something (http://reviews.llvm.org/D3240), but I haven't been
>>>> following
>>>> the patch.  I have no real urgency in seeing this enhancement, so
>>>> prioritize this as you see fit.
>>>>
>>>> Chad
>>>>
>>>>
>>>>
>>>>> Hi Chad,
>>>>>
>>>>> Ill see if its possible to narrow the scope. We might be able to
>>>>> mark
>>>>> the functions that require special handling as musttail instead of
>>>>> tail and return false in FastISel::SelectInstruction upon seeing
>>>>> those
>>>>> functions.
>>>>>
>>>>>
>>>>>> On Aug 13, 2014, at 4:55 PM, Chad Rosier <mcrosier at codeaurora.org>
>>>>>> wrote:
>>>>>>
>>>>>> Hi Akira,
>>>>>> Would it be possible to narrow the scope to only those tail calls
>>>>>> that
>>>>>> require special handling?  Or is it that the front-end will only
>>>>>> emit
>>>>>> tail
>>>>>> calls for those calls that require it at -O0?  Please clarify.
>>>>>>
>>>>>> Chad
>>>>>>
>>>>>>> Author: ahatanak
>>>>>>> Date: Wed Aug 13 18:23:58 2014
>>>>>>> New Revision: 215600
>>>>>>>
>>>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=215600&view=rev
>>>>>>> Log:
>>>>>>> [AArch64, fast-isel] Fall back to SelectionDAG to select tail
>>>>>>> calls.
>>>>>>>
>>>>>>> Certain functions such as objc_autoreleaseReturnValue have to be
>>>>>>> called
>>>>>>> as
>>>>>>> tail-calls even at -O0. Since normal fast-isel doesn't emit calls
>>>>>>> as
>>>>>>> tail
>>>>>>> calls,
>>>>>>> we have to fall back to SelectionDAG to select calls that are
>>>>>>> marked
>>>>>>> as
>>>>>>> tail.
>>>>>>>
>>>>>>> <rdar://problem/17991614>
>>>>>>>
>>>>>>>
>>>>>>> Added:
>>>>>>>  llvm/trunk/test/CodeGen/AArch64/tailcall-fastisel.ll
>>>>>>> Modified:
>>>>>>>  llvm/trunk/lib/Target/AArch64/AArch64FastISel.cpp
>>>>>>>  llvm/trunk/test/CodeGen/AArch64/arm64-abi_align.ll
>>>>>>>
>>>>>>> Modified: llvm/trunk/lib/Target/AArch64/AArch64FastISel.cpp
>>>>>>> URL:
>>>>>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Target/AArch64/AArch64FastISel.cpp?rev=215600&r1=215599&r2=215600&view=diff
>>>>>>> ==============================================================================
>>>>>>> --- llvm/trunk/lib/Target/AArch64/AArch64FastISel.cpp (original)
>>>>>>> +++ llvm/trunk/lib/Target/AArch64/AArch64FastISel.cpp Wed Aug 13
>>>>>>> 18:23:58
>>>>>>> 2014
>>>>>>> @@ -1687,10 +1687,15 @@ bool AArch64FastISel::FinishCall(CallLow
>>>>>>>
>>>>>>> bool AArch64FastISel::FastLowerCall(CallLoweringInfo &CLI) {
>>>>>>> CallingConv::ID CC  = CLI.CallConv;
>>>>>>> +  bool IsTailCall     = CLI.IsTailCall;
>>>>>>> bool IsVarArg       = CLI.IsVarArg;
>>>>>>> const Value *Callee = CLI.Callee;
>>>>>>> const char *SymName = CLI.SymName;
>>>>>>>
>>>>>>> +  // Allow SelectionDAG isel to handle tail calls.
>>>>>>> +  if (IsTailCall)
>>>>>>> +    return false;
>>>>>>> +
>>>>>>> CodeModel::Model CM = TM.getCodeModel();
>>>>>>> // Only support the small and large code model.
>>>>>>> if (CM != CodeModel::Small && CM != CodeModel::Large)
>>>>>>>
>>>>>>> Modified: llvm/trunk/test/CodeGen/AArch64/arm64-abi_align.ll
>>>>>>> URL:
>>>>>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/arm64-abi_align.ll?rev=215600&r1=215599&r2=215600&view=diff
>>>>>>> ==============================================================================
>>>>>>> --- llvm/trunk/test/CodeGen/AArch64/arm64-abi_align.ll (original)
>>>>>>> +++ llvm/trunk/test/CodeGen/AArch64/arm64-abi_align.ll Wed Aug 13
>>>>>>> 18:23:58
>>>>>>> 2014
>>>>>>> @@ -511,7 +511,9 @@ entry:
>>>>>>> ; CHECK: str {{w[0-9]+}}, [sp]
>>>>>>> ; FAST-LABEL: i64_split
>>>>>>> ; FAST: ldr x7, [{{x[0-9]+}}]
>>>>>>> -; FAST: str {{w[0-9]+}}, [sp]
>>>>>>> +; FAST: mov x[[R0:[0-9]+]], sp
>>>>>>> +; FAST: orr w[[R1:[0-9]+]], wzr, #0x8
>>>>>>> +; FAST: str w[[R1]], {{\[}}x[[R0]]{{\]}}
>>>>>>> %0 = load i64* bitcast (%struct.s41* @g41 to i64*), align 16
>>>>>>> %call = tail call i32 @callee_i64(i32 1, i32 2, i32 3, i32 4, i32
>>>>>>> 5,
>>>>>>>                                   i32 6, i32 7, i64 %0, i32 8) #5
>>>>>>>
>>>>>>> Added: llvm/trunk/test/CodeGen/AArch64/tailcall-fastisel.ll
>>>>>>> URL:
>>>>>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/test/CodeGen/AArch64/tailcall-fastisel.ll?rev=215600&view=auto
>>>>>>> ==============================================================================
>>>>>>> --- llvm/trunk/test/CodeGen/AArch64/tailcall-fastisel.ll (added)
>>>>>>> +++ llvm/trunk/test/CodeGen/AArch64/tailcall-fastisel.ll Wed Aug 13
>>>>>>> 18:23:58 2014
>>>>>>> @@ -0,0 +1,11 @@
>>>>>>> +; RUN: llc < %s -mtriple=arm64-apple-darwin -O0 | FileCheck %s
>>>>>>> +
>>>>>>> +; CHECK: b _foo0
>>>>>>> +
>>>>>>> +define i32 @foo1() {
>>>>>>> +entry:
>>>>>>> +  %call = tail call i32 @foo0()
>>>>>>> +  ret i32 %call
>>>>>>> +}
>>>>>>> +
>>>>>>> +declare i32 @foo0()
>>>>>>>
>>>>>>>
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>>>>>>>
>>>>>>
>>>>>>
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>>>>>
>>>>>
>>>>
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