llvm dev - Mar 2019 - How to use bugpoint for backend native code generation?

Hello,

I’m writing machine function passes for programs running with ARMv7-M ISA. My
pass adds some instructions to the original code, and now I got

fatal error: error in backend: out of range pc-relative fixup value

when llvm generates native code after running the transformation pass. I think
it’s because my transformation pass increases the size of the original code and
thus some branch instructions cannot reach their destination (for example, the
beq instruction can only jump to a pc-relative destination within range -256 to
254 bytes).

It looks like bugpoint is a good tool to pinpoint which branch instruction
caused the error. Unfortunately the blogs I found online are all about using
bugpoint for IR passes but not for native code generation. Can anyone give me
some help on this issue? (pointing to a good tutorial, or sharing some
techniques to pinpoint the troublesome instructions, etc.)

Thanks very much.

- Jie
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Currently bugpoint doesn’t support backend passes.  We didn’t have any support
for serializing MachineFunctions until relatively recently, and that support
still isn’t really complete.  Depending on the nature of your issue,
opt-bisect-limit might be helpful; see http://llvm.org/docs/OptBisect.html .

That said, there’s probably a more straightforward way to track down your
particular issue: you should be able to get a readable error message by just
emitting assembly, then feeding the output to clang.  And my first guess for the
cause of the problem is that your pass is running after ARMConstantIslands,
which is almost never what you want.

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org> On Behalf Of Jie Zhou
via llvm-dev
Sent: Monday, March 25, 2019 3:49 PM
To: via llvm-dev <llvm-dev at lists.llvm.org>
Subject: [EXT] [llvm-dev] How to use bugpoint for backend native code
generation?

Hello,

I’m writing machine function passes for programs running with ARMv7-M ISA. My
pass adds some instructions to the original code, and now I got

fatal error: error in backend: out of range pc-relative fixup value

when llvm generates native code after running the transformation pass. I think
it’s because my transformation pass increases the size of the original code and
thus some branch instructions cannot reach their destination (for example, the
beq instruction can only jump to a pc-relative destination within range -256 to
254 bytes).

It looks like bugpoint is a good tool to pinpoint which branch instruction
caused the error. Unfortunately the blogs I found online are all about using
bugpoint for IR passes but not for native code generation. Can anyone give me
some help on this issue? (pointing to a good tutorial, or sharing some
techniques to pinpoint the troublesome instructions, etc.)

Thanks very much.

- Jie
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On Mar 25, 2019, at 19:27, Eli Friedman <efriedma at
quicinc.com<mailto:efriedma at quicinc.com>> wrote:

Currently bugpoint doesn’t support backend passes.  We didn’t have any support
for serializing MachineFunctions until relatively recently, and that support
still isn’t really complete.  Depending on the nature of your issue,
opt-bisect-limit might be helpful; see
http://llvm.org/docs/OptBisect.html<https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_docs_OptBisect.html&d=DwMGaQ&c=kbmfwr1Yojg42sGEpaQh5ofMHBeTl9EI2eaqQZhHbOU&r=KAtyTEI8n3FritxDpKpR7rv3VjdmUs0luiVKZLb_bNI&m=N8T9xou8gKEMYjs_z4t_ajAvxVNR0Gi7DSTGkmK3UMU&s=i7-oSWHTWDhAbbHVN9ADDhdRRvguklobURQzSWuFVWM&e=>
.

That said, there’s probably a more straightforward way to track down your
particular issue: you should be able to get a readable error message by just
emitting assembly, then feeding the output to clang.  And my first guess for the
cause of the problem is that your pass is running after ARMConstantIslands,
which is almost never what you want.

Hi Eli,

Can you be more specific on “emitting assembly”? I know how to generate asm code
from source code, but is there a way to generate native asm code after running
machine function passes?

- Jie

-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces
at lists.llvm.org>> On Behalf Of Jie Zhou via llvm-dev
Sent: Monday, March 25, 2019 3:49 PM
To: via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>>
Subject: [EXT] [llvm-dev] How to use bugpoint for backend native code
generation?

Hello,

I’m writing machine function passes for programs running with ARMv7-M ISA. My
pass adds some instructions to the original code, and now I got

fatal error: error in backend: out of range pc-relative fixup value

when llvm generates native code after running the transformation pass. I think
it’s because my transformation pass increases the size of the original code and
thus some branch instructions cannot reach their destination (for example, the
beq instruction can only jump to a pc-relative destination within range -256 to
254 bytes).

It looks like bugpoint is a good tool to pinpoint which branch instruction
caused the error. Unfortunately the blogs I found online are all about using
bugpoint for IR passes but not for native code generation. Can anyone give me
some help on this issue? (pointing to a good tutorial, or sharing some
techniques to pinpoint the troublesome instructions, etc.)

Thanks very much.

- Jie

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On Mar 25, 2019, at 19:27, Eli Friedman <efriedma at
quicinc.com<mailto:efriedma at quicinc.com>> wrote:

Currently bugpoint doesn’t support backend passes.  We didn’t have any support
for serializing MachineFunctions until relatively recently, and that support
still isn’t really complete.  Depending on the nature of your issue,
opt-bisect-limit might be helpful; see
http://llvm.org/docs/OptBisect.html<https://urldefense.proofpoint.com/v2/url?u=http-3A__llvm.org_docs_OptBisect.html&d=DwMGaQ&c=kbmfwr1Yojg42sGEpaQh5ofMHBeTl9EI2eaqQZhHbOU&r=KAtyTEI8n3FritxDpKpR7rv3VjdmUs0luiVKZLb_bNI&m=N8T9xou8gKEMYjs_z4t_ajAvxVNR0Gi7DSTGkmK3UMU&s=i7-oSWHTWDhAbbHVN9ADDhdRRvguklobURQzSWuFVWM&e=>
.

That said, there’s probably a more straightforward way to track down your
particular issue: you should be able to get a readable error message by just
emitting assembly, then feeding the output to clang.  And my first guess for the
cause of the problem is that your pass is running after ARMConstantIslands,
which is almost never what you want.

Hi Eli,

Just an FYI, I tried to run my pass before ARMConstantIslandPass rather than
after it, and it solved my problem, although I’m not very clear of why it works.

Thanks,
- Jie


-Eli

From: llvm-dev <llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces
at lists.llvm.org>> On Behalf Of Jie Zhou via llvm-dev
Sent: Monday, March 25, 2019 3:49 PM
To: via llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at
lists.llvm.org>>
Subject: [EXT] [llvm-dev] How to use bugpoint for backend native code
generation?

Hello,

I’m writing machine function passes for programs running with ARMv7-M ISA. My
pass adds some instructions to the original code, and now I got

fatal error: error in backend: out of range pc-relative fixup value

when llvm generates native code after running the transformation pass. I think
it’s because my transformation pass increases the size of the original code and
thus some branch instructions cannot reach their destination (for example, the
beq instruction can only jump to a pc-relative destination within range -256 to
254 bytes).

It looks like bugpoint is a good tool to pinpoint which branch instruction
caused the error. Unfortunately the blogs I found online are all about using
bugpoint for IR passes but not for native code generation. Can anyone give me
some help on this issue? (pointing to a good tutorial, or sharing some
techniques to pinpoint the troublesome instructions, etc.)

Thanks very much.

- Jie

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