[llvm-dev] Register allocator behaves differently when compiling with and without -g

[David Blaikie via llvm-dev]

On Thu, Feb 2, 2017 at 9:18 AM Stephen Rogers <stephen.rogers at movidius.com>
wrote:

> Turns out this was being caused by our own pre-register allocation
> instruction scheduler. The debug information caused calls to
> LiveIntervals::handleMove to be ordered slightly differently. This change
> meant a range of slot indexes had to be renumbered.
>
> I'm curious though. Despite the order of instructions being identical in
> both cases, the weights assigned to virtual registers was changed by the
> different slot indexes. Why does the register allocator use the absolute
> values of the slot indexes in this way when they don't seem to correspond
> directly to the structure of the incoming code?
>

Can't say I know too much about the register allocator or how that might be
happening (so I added Lang Hames who might have some context).


>
> Thanks,
> Stephen
>
> On 2 February 2017 at 17:03, Robinson, Paul via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> I remember running into cases where –g introduces pseudo-instructions for
> .cfi-* directives, and these become scheduling barriers and therefore
> affect the final code a bit; I don't remember anything about register
> allocation being affected, though.
>
> A reproducer would be awesome.
>
> --paulr
>
>
>
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of *Adrian
> Prantl via llvm-dev
> *Sent:* Thursday, February 02, 2017 8:46 AM
> *To:* David Blaikie
> *Cc:* llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] Register allocator behaves differently when
> compiling with and without -g
>
>
>
>
>
> On Feb 2, 2017, at 8:20 AM, David Blaikie <dblaikie at gmail.com> wrote:
>
>
>
> The goal/intent is that debug info does not affect code generation. There
> are (many?) bugs. I think Apple folks (cc'd Adrian) may be looking at this
> a bit recently, not sure.
>
> The fixes aren't usually too invasive (usually involve something counting
> instructions where it needs to skip counting debug intrinsics), if you're
> interested in having a go at fixing it yourself & sending a patch for
> review.
>
>
>
> Just wanted to add, if you are not able to fix it yourself, it would also
> be helpful if you could file a PR with a testcase.
>
>
>
> -- adrian
>
>
>
>
>
> On Thu, Feb 2, 2017 at 8:18 AM Stephen Rogers via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> Hi all,
>
> In several of our tests, I have noticed that the register allocator
> allocates to virtual registers in a different order when compiling with the
> clang option -g. Before entering the register allocator, the code is
> identical when compiling with and without -g (with the exception of "
> DBG_VALUE" instructions). The only difference I can see is the value
> assigned to the slot index for each instruction. As an example, without -g
> a snippet of a basic block looks like this:
>
> 32B        %vreg29<def> = LDImm 1; REG1:%vreg29
> 36B        %vreg44<def> = LDImm 1103515245; REG1:%vreg44
> 40B        %vreg143:vsub32_1<def,read-undef> = LDImm 0; REG2:%vreg143
> 44B        %vreg68<def> = LDImm 12345; REG1:%vreg68
> 64B        %vreg143:vsub32_0<def> = COPY %vreg143:vsub32_1; REG2:%vreg143
> 72B        %vreg78<def> = LDImm 32; REG1:%vreg78
>
> But when I specify -g, this becomes:
>
> 32B        %vreg29<def> = LDImm 1; REG1:%vreg29 dbg:path/to/source:9:34 @[
> path/to/source:25:13 ]
> 36B        %vreg44<def> = LDImm 1103515245; REG1:%vreg44
> dbg:path/to/source:9:34 @[ path/to/source:25:13 ]
> 44B        %vreg143:vsub32_1<def,read-undef> = LDImm 0; REG2:%vreg143
> 48B        %vreg68<def> = LDImm 12345; REG1:%vreg68
> dbg:path/to/source:9:47 @[ path/to/source:25:13 ]
> 56B        %vreg143:vsub32_0<def> = COPY %vreg143:vsub32_1; REG2:%vreg143
> 60B        %vreg78<def> = LDImm 32; REG1:%vreg78
>
> This change seems to affect the weight assigned to the LiveIntervals for
> each virtual register which in turn changes the allocation order.
>
> It would be my expectation that LLVM should generate identical code for a
> source file with and without debug information enabled. Is it possible we
> are missing some hook in our target backend that would prevent this? Or is
> this the expected behaviour in LLVM?
>
> Thanks,
>
> Stephen
>
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