[llvm-dev] [AArch64] Generated assembly differs depending on whether debug information is generated or not

David Tellenbach via llvm-dev
2019-10-05T14:03:30Z

Hi Vedant,

thanks for your answer and sorry for the late response.
It seems like D68076 might not address the underlying issue here (e.g. it
probably doesn't improve the situation for projects using `-g
-fno-unwind-tables`?).
Yes, D68075 is a somewhat conservative patch that aligns the behaviour on
AArch64 (for GNU targets) that leads to consistent generated assembly. As you
said it does not help if unwind tables are explicitly disabled
(`-fno-unwind-tables`). It is a conservative patch since it decreases scheduling
potential (due to smaller scheduling regions) for the non-debug case but fixes
the bug of generating inconsistent assembly.
Would you mind elaborating a bit on your proposals to delay/change CFI
instruction insertion? In particular, it'd help to hear a bit about how CFI
instructions are inserted today (is some of it done by CFIInstrInserter, and the
rest by target-specific frame lowering code?).
CFI instructions are inserted during target specific frame lowering, the
CFIInstrInserter is only run on X86 targets and seems to verify the correctness
of CFI instructions after the got inserted during X86 frame lowering.

Regarding the 3 possible roadmaps to solve the issue (see my first email) I
currently think 3 (changing instruction scheduling such that CFI instructions
are scheduled together with stack altering instructions) is the most promising
one because it wouldn't require targets specific changes. Since e.g. X86 or
if I remember correctly also AArch64 on Darwin targets insert CFI instructions
in both, debug and non-debug mode, solution 3 would increase scheduling
potential for these targets.

To summarize: D68075 would align non-debug mode with debug mode and therefore
potentially *decrease* scheduling potential. Solution 3 would align debug mode
with non-debug mode (in terms of instruction scheduling) and therefore
*increase* scheduling potential.

    David
________________________________
From: vsk at apple.com <vsk at apple.com> on behalf of Vedant Kumar
<vedant_kumar at apple.com>
Sent: 30 September 2019 20:50
To: David Tellenbach <David.Tellenbach at arm.com>
Cc: paul.robinson at sony.com <paul.robinson at sony.com>; llvm-dev at
lists.llvm.org <llvm-dev at lists.llvm.org>; nd <nd at arm.com>; Tim
Northover <tnorthover at apple.com>; Ahmed Bougacha <abougacha at
apple.com>
Subject: Re: [llvm-dev] [AArch64] Generated assembly differs depending on
whether debug information is generated or not

Hi David,

Thanks for looking into this.

It seems like D68076 might not address the underlying issue here (e.g. it
probably doesn't improve the situation for projects using `-g
-fno-unwind-tables`?).

Would you mind elaborating a bit on your proposals to delay/change CFI
instruction insertion? In particular, it'd help to hear a bit about how CFI
instructions are inserted today (is some of it done by CFIInstrInserter, and the
rest by target-specific frame lowering code?).

best,
vedant

On Sep 26, 2019, at 6:57 AM, David Tellenbach via llvm-dev <llvm-dev at
lists.llvm.org<mailto:llvm-dev at lists.llvm.org>> wrote:

Hi Paul,

thanks for your comments.
This is PR37240 (https://bugs.llvm.org/show_bug.cgi?id=37240).  I suspect this
problem affects all targets; your patch D68076 would address it only for
AArch64.  Although I would suggest you do some careful measurements to determine
the runtime performance effect, to decide whether this is acceptable.
Yes, in principle the problem that instruction scheduling is dependent on the
presence of cfi instruction should affect more targets than AArch64. However,
this does not imply that all of these targets produce inconsistent assembly
depending on debug information.

The more complete approach in your steps 2 + 3 would solve this for all targets,
assuming the solution did not have to be very target-specific.  This would
benefit the entire community.
At least 2. would require a lot of target dependent changes because the
insertion of cfi instructions would have to be moved from target specific frame
lowering into an (probably again target specific) insertion pass.

        David

On 26/09/2019 13:55, paul.robinson at sony.com<mailto:paul.robinson at
sony.com> wrote:
Hi David,

This is PR37240 (https://bugs.llvm.org/show_bug.cgi?id=37240).  I suspect this
problem affects all targets; your patch D68076 would address it only for
AArch64.  Although I would suggest you do some careful measurements to determine
the runtime performance effect, to decide whether this is acceptable.

The more complete approach in your steps 2 + 3 would solve this for all targets,
assuming the solution did not have to be very target-specific.  This would
benefit the entire community.
--paulr

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] On Behalf Of David
Tellenbach via llvm-dev
Sent: Thursday, September 26, 2019 5:57 AM
To: llvm-dev
Cc: nd
Subject: [llvm-dev] [AArch64] Generated assembly differs depending on whether
debug information is generated or not

Hi,

we at Arm have noticed that assembly can differ when compiling for AArch64
depending on whether debug information is generated or not.

The issue is reproducible for the following small example compiled with `-O1`
for `aarch64-arm-linux-gnu`:

    a() {
      b(a);
      for (;;)
        c("", b);
    }

The reason for the difference is that AArch64 frame lowering emits CFI
instructions if debug information is enabled but not if not. CFI instructions
act as scheduling boundaries during instruction scheduling and therefore lead to
differing scheduling regions and an overall different instruction scheduling.

We see several ways to fix the issue and would welcome comments on this:

  1. Enabling unwind tables by default for AArch64: By enabling unwind tables
     by default CFI instructions will be inserted in both, debug and non-debug
     mode. This should lead to smaller scheduling regions and probably to less
     scheduling potential.

     However, I've measured the average size of scheduling regions for
randomly
     generated programs with and without default unwind tables and found an
     average difference of 0.5 to 1 instruction. Other architectures such as x86
     do exactly this and therefore don't face the issue.

     The following patch on Phabricator introduces the said change:
                    https://reviews.llvm.org/D68076

  2. Postpone insertion of CFI instructions until after instruction scheduling.
     This would require a new pass running after instruction scheduling that
     inserts CFI instructions if needed. The downside I see is increased
     compile-time and probably some code duplication with frame lowering.

  3. Change instruction scheduling such that CFI instructions get tied together
     with relevant instructions in such a way that they get scheduled together.
     If this could work it would probably the cleanest solution.

To summarize:
1. would make scheduling in the non-debug case behave like in the
debug case and therefore probably cost some scheduling potential. However, it
would be by far the most easy to implement. 2. + 3. would probably lead to
better scheduling but seem to be more complex to implement.

Comments and additional ideas are welcome.

    David


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