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

Thu Feb 2 09:03:10 PST 2017

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<mailto: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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