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

[Bob Wilson]

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 aren’t worried about those cases, I guess narrowing down the scope of functions that are emitted as tail calls in AArch64’s fast-isel doesn’t 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,
>>>> 
>>>> I’ll see if it’s 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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>>>>>> 
>>>>> 
>>>>> 
>>>>> --
>>>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>>>>> hosted by The Linux Foundation
>>>>> 
>>>> 
>>>> 
>>> 
>>> 
>>> --
>>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>>> hosted by The Linux Foundation
>>> 
>>> 
>> 
>> 
>> -- 
>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>> hosted by The Linux Foundation
>> 
> 
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