llvm dev - May 2012 - [LLVMdev] register allocation problems in trunk with IMPLICIT_DEF

Hi,

Recently code using IMPLICIT_DEF and INSERT_SUBREG started to break:

            %vreg9<def> = IMPLICIT_DEF
            %vreg10<def> = INSERT_SUBREG %vreg9<kill>,
%vreg1<kill>, hi
            %vreg12<def> = sub %vreg10<kill>, %vreg11<kill>
=>
            %vreg10<def> = IMPLICIT_DEF
            %vreg10:hi<def> = COPY %vreg1<kill>
            %vreg12<def> = sub %vreg10<kill>, %vreg11<kill>
=>
            %vreg10:hi<def,read-undef> = COPY %vreg1<kill>,
%vreg10<imp-def>
            %vreg12<def> = sub %vreg10<kill>, %vreg11<kill>
=>
            %vreg10:hi<def,read-undef> = COPY %a1_h<kill>
            %vreg12<def> = sub %vreg10<kill>, %vreg11<kill>
=>
            %a2_h<def> = COPY %a1_h<kill>
            %a2<def> = sub %a2<kill>, %a3<kill>

*** Bad machine code: Using an undefined physical register ***
- function:    array
- basic block:  0x43ac7d0 (BB#0)
- instruction: %a2<def> = sub %a2<kill>, %a3<kill
- operand 1:   %a2<kill>

This all worked well before. Is there a change somewhere in the framework that
will make the code on top wrong?

Jonas
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On May 9, 2012, at 6:27 AM, Jonas Paulsson <jonas.paulsson at
ericsson.com> wrote:
> Hi,
>  
> Recently code using IMPLICIT_DEF and INSERT_SUBREG started to break:
>  
>             %vreg9<def> = IMPLICIT_DEF
>             %vreg10<def> = INSERT_SUBREG %vreg9<kill>,
%vreg1<kill>, hi
>             %vreg12<def> = sub %vreg10<kill>,
%vreg11<kill>
> =>
>             %vreg10<def> = IMPLICIT_DEF
>             %vreg10:hi<def> = COPY %vreg1<kill>
>             %vreg12<def> = sub %vreg10<kill>,
%vreg11<kill>
> =>
>             %vreg10:hi<def,read-undef> = COPY %vreg1<kill>,
%vreg10<imp-def>
>             %vreg12<def> = sub %vreg10<kill>,
%vreg11<kill>
> =>
>             %vreg10:hi<def,read-undef> = COPY %a1_h<kill>
>             %vreg12<def> = sub %vreg10<kill>,
%vreg11<kill>
> =>
>             %a2_h<def> = COPY %a1_h<kill>
>             %a2<def> = sub %a2<kill>, %a3<kill>
>  
> *** Bad machine code: Using an undefined physical register ***
> - function:    array
> - basic block:  0x43ac7d0 (BB#0)
> - instruction: %a2<def> = sub %a2<kill>, %a3<kill
> - operand 1:   %a2<kill>
>  
> This all worked well before. Is there a change somewhere in the framework
that will make the code on top wrong?
This looks like a bug in the register allocator you are using. There should have
been an %a2<imp-def> operand added to the copy instruction when rewriting
virtual registers to physical registers.

Please file a PR, preferably with a test case for an in-target tree. Don't
forget to state which register allocator you are using.

/jakob

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I used llvm-stress to find a similar problem on x86-64. See
http://llvm.org/bugs/show_bug.cgi?id=12821.

BTW, llvm-stress is a great tool!

/Patrik Hägglund
________________________________

From: llvmdev-bounces at cs.uiuc.edu [mailto:llvmdev-bounces at cs.uiuc.edu] On
Behalf Of Jakob Stoklund Olesen
Sent: den 9 maj 2012 18:21
To: Jonas Paulsson
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] register allocation problems in trunk with IMPLICIT_DEF



On May 9, 2012, at 6:27 AM, Jonas Paulsson <jonas.paulsson at
ericsson.com> wrote:


	Hi,
	 
	Recently code using IMPLICIT_DEF and INSERT_SUBREG started to break:
	 
	            %vreg9<def> = IMPLICIT_DEF
	            %vreg10<def> = INSERT_SUBREG %vreg9<kill>,
%vreg1<kill>, hi
	            %vreg12<def> = sub %vreg10<kill>, %vreg11<kill>
	=>
	            %vreg10<def> = IMPLICIT_DEF
	            %vreg10:hi<def> = COPY %vreg1<kill>
	            %vreg12<def> = sub %vreg10<kill>, %vreg11<kill>
	=>
	            %vreg10:hi<def,read-undef> = COPY %vreg1<kill>,
%vreg10<imp-def>
	            %vreg12<def> = sub %vreg10<kill>, %vreg11<kill>
	=>
	            %vreg10:hi<def,read-undef> = COPY %a1_h<kill>
	            %vreg12<def> = sub %vreg10<kill>, %vreg11<kill>
	=>
	            %a2_h<def> = COPY %a1_h<kill>
	            %a2<def> = sub %a2<kill>, %a3<kill>
	 
	*** Bad machine code: Using an undefined physical register ***
	- function:    array
	- basic block:  0x43ac7d0 (BB#0)
	- instruction: %a2<def> = sub %a2<kill>, %a3<kill
	- operand 1:   %a2<kill>
	 
	This all worked well before. Is there a change somewhere in the framework that
will make the code on top wrong?


This looks like a bug in the register allocator you are using. There should have
been an %a2<imp-def> operand added to the copy instruction when rewriting
virtual registers to physical registers.

Please file a PR, preferably with a test case for an in-target tree. Don't
forget to state which register allocator you are using.

/jakob