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

Adrian Prantl via llvm-dev llvm-dev at lists.llvm.org
Thu Feb 2 08:46:30 PST 2017

> 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 <mailto: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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