llvm dev - Jun 2010 - [LLVMdev] Unused argument registers can not be reused ?

While migrating my codebase from llvm-2.6 to llvm-2.7, I found a different
behaviour in the register allocation. I have been able to reproduce it using the
msp430 backend, with the 2.7 release as well as the svn head.

For the msp430, the first four parameters of a function are passed thru
registers. What I observe is that if those parameters are not used inside the
function, those registers can not be used.

For example, with the following C code:
> cat test_unused_regs.c
short a = 2;
short b = 32;
short r = 1024;

void
test(short w, short x, short y, short z)
{
  r -= a+b;
}

I get the following assembly code for the test function after running thru
clang:
> clang -ccc-host-triple msp430-unknown-unknown -O2 -fomit-frame-pointer -S
-o test_unused_regs.s test_unused_regs.c
> cat test_unused_regs.s
...
test:
      push.w      r11
      mov.w &b, r11
      add.w &a, r11
      sub.w r11, &r
      pop.w r11
      ret
...

Instead of using r11, llvm should have used r12/r13/r14 or r15. This was my
expectation, and the behaviour with llvm-2.6. This would have spared
saving/restoring r11.

Although this is not a functionnal regression, this looks to me like a
performance regression, unless the calling convention has changed (i.e. the
arguments can not be clobbered anymore).

I have not yet understood why, but the liveIntervals analysis dump looks dubious
to me (R12W,R13W,R14W and R15W should be dead/killed livein registers)
:
> llc -march=msp430 -debug-only=liveintervals -o test_unused_regs.s
test_unused_regs.ll
********** COMPUTING LIVE INTERVALS **********
********** Function: test
BB#0:       # derived from entry
            livein register: R15W live through +[0,40:0)
            livein register: R15B dead +[0,3:0)
            livein register: R14W live through +[0,40:0)
            livein register: R14B dead +[0,3:0)
            livein register: R13W live through +[0,40:0)
            livein register: R13B dead +[0,3:0)
            livein register: R12W live through +[0,40:0)
            livein register: R12B dead +[0,3:0)
4     %reg1028<def> = MOV16rm %reg0, <ga:@b>; mem:LD2[@b]
            register: %reg1028 +[6,14:0)
12    %reg1029<def> = MOV16rr %reg1028<kill>
            register: %reg1029 +[14,30:0)
20    %reg1029<def> = ADD16rm %reg1029, %reg0, <ga:@a>,
%SRW<imp-def>; mem:LD2[@a]
            register: %reg1029 replace range with [14,22:1) RESULT:
%reg1029,0.000000e+00 = [14,22:1)[22,30:0)  0 at 22-(30) 1 at 14-(22)
28    SUB16mr %reg0, <ga:@r>, %reg1029<kill>, %SRW<imp-def>;
mem:ST2[@r] LD2[@r]
40    RET

Any help or hints would be much welcome !

Best regards,
--
Arnaud de Grandmaison
-------------- next part --------------
An HTML attachment was scrubbed...
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20100603/1db8a188/attachment.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: test_unused_regs.ll
Type: application/octet-stream
Size: 909 bytes
Desc: test_unused_regs.ll
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20100603/1db8a188/attachment.obj>
-------------- next part --------------
An embedded and charset-unspecified text was scrubbed...
Name: test_unused_regs.c
URL:
<http://lists.llvm.org/pipermail/llvm-dev/attachments/20100603/1db8a188/attachment.c>

On Jun 3, 2010, at 6:33 AM, Arnaud Allard de Grandmaison wrote:
> While migrating my codebase from llvm-2.6 to llvm-2.7, I found a different
behaviour in the register allocation. I have been able to reproduce it using the
msp430 backend, with the 2.7 release as well as the svn head.
>  
> For the msp430, the first four parameters of a function are passed thru
registers. What I observe is that if those parameters are not used inside the
function, those registers can not be used.
It looks like you found a bug, R12-R15 should be usable inside the function.

Please file a PR against the code generator.

I think that unused arguments shouldn't have been marked live in at all.

Thanks,
/jakob

Oops. Forgot the mailing list in the reply.
--
Arnaud de Grandmaison
________________________________________
From: Arnaud Allard de Grandmaison
Sent: Thursday, June 03, 2010 9:57 PM
To: Jakob Stoklund Olesen
Subject: RE: [LLVMdev] Unused argument registers can not be reused ?

I have finally been able to make some progress : the bug has been
introduced/revealed by svn commit #95493, which enabled
DeadMachineInstructionElimPass for all targets.
Disabling this pass makes llvm-2.7 behave as expected.

What we can see when dumping the test function body at the path entry & exit
is :
# Machine code for function test:
Function Live Ins: %R15W in reg%1024, %R14W in reg%1025, %R13W in reg%1026,
%R12W in reg%1027

BB#0: derived from LLVM BB %entry
    Live Ins: %R15W %R14W %R13W %R12W
        %reg1027<def> = MOV16rr %R12W
        %reg1026<def> = MOV16rr %R13W
        %reg1025<def> = MOV16rr %R14W
        %reg1024<def> = MOV16rr %R15W
        %reg1028<def> = MOV16rm %reg0, <ga:@b>; mem:LD2[@b]
        %reg1029<def> = ADD16rm %reg1028, %reg0, <ga:@a>,
%SRW<imp-def,dead>; mem:LD2[@a]
        SUB16mr %reg0, <ga:@r>, %reg1029, %SRW<imp-def,dead>;
mem:ST2[@r] LD2[@r]
        RET

# End machine code for function test.

# Machine code for function test:
Function Live Ins: %R15W in reg%1024, %R14W in reg%1025, %R13W in reg%1026,
%R12W in reg%1027

BB#0: derived from LLVM BB %entry
    Live Ins: %R15W %R14W %R13W %R12W
        %reg1028<def> = MOV16rm %reg0, <ga:@b>; mem:LD2[@b]
        %reg1029<def> = ADD16rm %reg1028, %reg0, <ga:@a>,
%SRW<imp-def,dead>; mem:LD2[@a]
        SUB16mr %reg0, <ga:@r>, %reg1029, %SRW<imp-def,dead>;
mem:ST2[@r] LD2[@r]
        RET

# End machine code for function test.

This points to 2 different ways of fixing the issue :
1- Fix the backends to not lower unused arguments, or at least, not mark them as
liveins --- if possible.
2- Fix the pass

I will fill a bug tomorrow for this tomorrow.

Best regards,
--
Arnaud de Grandmaison
________________________________________
From: Jakob Stoklund Olesen [stoklund at 2pi.dk]
Sent: Thursday, June 03, 2010 7:39 PM
To: Arnaud Allard de Grandmaison
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Unused argument registers can not be reused ?

On Jun 3, 2010, at 6:33 AM, Arnaud Allard de Grandmaison wrote:
> While migrating my codebase from llvm-2.6 to llvm-2.7, I found a different
behaviour in the register allocation. I have been able to reproduce it using the
msp430 backend, with the 2.7 release as well as the svn head.
>
> For the msp430, the first four parameters of a function are passed thru
registers. What I observe is that if those parameters are not used inside the
function, those registers can not be used.
It looks like you found a bug, R12-R15 should be usable inside the function.

Please file a PR against the code generator.

I think that unused arguments shouldn't have been marked live in at all.

Thanks,
/jakob