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

David Blaikie via llvm-dev
Thu, 02 Feb 2017 16:20:21 -0000

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.

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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