[llvm] r215600 - [AArch64, fast-isel] Fall back to SelectionDAG to select tail calls.
Akira Hatanaka
ahatanaka at apple.com
Fri Aug 15 12:53:03 PDT 2014
My understanding is that strictly speaking it’s not necessary to tail call objc functions which clang marks as “tail”, but it’s very desirable to do so, even at -O0 (see the following commits).
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20121210/069653.html
http://lists.cs.uiuc.edu/pipermail/cfe-commits/Week-of-Mon-20140127/098177.html
As you’ve predicted, verifier asserts if I change those “tail” calls to “musttail” calls and run make check.
On Aug 15, 2014, at 11:38 AM, Reid Kleckner <rnk at google.com> wrote:
> Do you have more information about what kinds of things need to be tail called for correctness? Currently musttail has strict verifier requirements about function prototypes matching, which I doubt will work for objc_autoreleaseReturnValue.
>
> We could relax them in some cases, but I want to make sure that we can tell, based only on the LLVM IR, that it will be possible to lower the call as a tail call.
>
> There are two major cases to think about: caller cleanup conventions and callee cleanup conventions.
> In caller cleanup, we can do a tail call so long as the we are passing fewer arguments than the caller pushed. At the IR level, we don't really know which args will be in registers.
> In callee cleanup, we can always do a tail call *if* we're willing to move the return address, which makes codegen a lot harder.
>
> It sounds like you would have a problem if you have a void function on a i686 that needs to tail call objc_retainAutoreleaseReturnValue, which presumably takes one argument.
>
>
> On Thu, Aug 14, 2014 at 4:09 PM, Bob Wilson <bob.wilson at apple.com> wrote:
> 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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> >>>>>
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> >>>>>
> >>>>
> >>>>
> >>>
> >>>
> >>> --
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> >>>
> >>
> >>
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