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

[Akira Hatanaka]

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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>>>>> 
>>>> 
>>>> 
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>>> 
>>> 
>> 
>> 
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>> Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum,
>> hosted by The Linux Foundation
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>> 
> 
> 
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