[PATCH] D11243: [ARM] Force LR spill in thumb2 noreturn functions

[Quentin Colombet]

Hi Bruno,

I guess that it is general goodness to have the right stack trace when you debug something, but I thought noreturn + nounwind allow the compiler to trash the information for optimization purposes?

I am not saying that your patch is wrong, I am pointing out that it seems we regress some pattern to improve the debug experience, and I am not sure this is something we should do.

My 2c.

Cheers,
-Quentin

> On Jul 15, 2015, at 2:59 PM, Bruno Cardoso Lopes <bruno.cardoso at gmail.com> wrote:
> 
> bruno created this revision.
> bruno added reviewers: t.p.northover, MatzeB.
> bruno added a subscriber: llvm-commits.
> bruno set the repository for this revision to rL LLVM.
> Herald added subscribers: rengolin, aemerson.
> 
>    In ARM mode we do an opt in noreturn functions that uses branch
>    instead of calls and explicitly save the pc (r151645):
> 
>      mov lr, pc
>      b _bar
> 
>    This isn't possible in thumb2 mode because the mode bit doesnt get set
>    (r167657). However, we currently emit a branch and link but do not save
>    the LR, which leads to bad backtraces coming from noreturn functions.
> 
>    Example:
> 
>    define void @a() noreturn nounwind ssp {
>      tail call void @b()
>      unreachable
>    }
>    define void @b() noreturn nounwind ssp {
>      tail call void @abort() noreturn nounwind
>      unreachable
>    }
>    declare void @abort() noreturn
> 
>    Currently generates:
> 
>    _a:
>            bl      _b
>    _b:
>            push    {r7, lr}
>            mov     r7, sp
>            blx     _abort
> 
>    Leading to bad backtraces. This patch generates instead:
> 
>    _a:
>            push    {r7, lr}
>            mov     r7, sp
>            bl      _b
>    _b:
>            push    {r7, lr}
>            mov     r7, sp
>            blx     _abort
> 
> Repository:
>  rL LLVM
> 
> http://reviews.llvm.org/D11243
> 
> Files:
>  lib/Target/ARM/ARMFrameLowering.cpp
>  lib/Target/ARM/ARMISelLowering.cpp
>  lib/Target/ARM/ARMMachineFunctionInfo.h
>  test/CodeGen/Thumb2/call-noret-direct.ll
> 
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