[llvm-dev] [DbgInfo] Potential bug in location list address ranges

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As Adrian said, we'd need to see the source of foo() to assess what the location-list for bar ought to be.
Without actually going to look, I would guess that 'poplt' is considered a conditional move, therefore r4's contents are not guaranteed after it executes (i.e. it is a clobber).  If one operand of 'poplt' is 'pc' then of course it is also a conditional indirect branch (which is probably but not necessarily a return).  This combination might be worth handling differently for location-list purposes.
But this is a tricky area, and we'd need to consider the consequences carefully.
--paulr

From: aprantl at apple.com [mailto:aprantl at apple.com]
Sent: Friday, April 27, 2018 11:22 AM
To: Son Tuan VU
Cc: Robinson, Paul; Vedant Kumar; dblaikie at gmail.com; llvm-dev
Subject: Re: [DbgInfo] Potential bug in location list address ranges




On Apr 27, 2018, at 7:48 AM, Son Tuan VU <sontuan.vu119 at gmail.com<mailto:sontuan.vu119 at gmail.com>> wrote:

Hi all,

Consider this ARM assembly code of a C function:

00008124 <foo>:
    8124:                   push    {r4, r6, r7, lr}
    8126:                   add     r7, sp, #8
    8128:                   mov     r4, r0
    812a:                   ldrsb.w r0, [r2]
    812e:                   cmp     r0, #1
    8130:                   itt     lt
    8132:                   movlt   r0, #85 ; 0x55
    8134:                   poplt   {r4, r6, r7, pc}            // a function return

    8136:                   ldrb.w  ip, [r1, #3]
    813a:                   ldrb.w  lr, [r4, #3]
    813e:                   movs    r0, #85 ; 0x55
    8140:                   cmp     lr, ip
    8142:                   bne.n   8168 <foo+0x44>

    8144:                   ldrb.w  ip, [r1, #2]
    8148:                   ldrb    r3, [r4, #2]
    814a:                   cmp     r3, ip
    814c:                   it      ne
    814e:                   popne   {r4, r6, r7, pc}          // a function return

    8150:                   ldrb.w  ip, [r1, #1]
    8154:                   ldrb    r3, [r4, #1]
    8156:                   cmp     r3, ip
    8158:                   bne.n   8168 <foo+0x44>

    815a:                   ldrb    r1, [r1, #0]
    815c:                   ldrb    r3, [r4, #0]
    815e:                   cmp     r3, r1
    8160:                   ittt    eq
    8162:                   moveq   r0, #3
    8164:                   strbeq  r0, [r2, #0]
    8166:                   moveq   r0, #170        ; 0xaa
    8168:                   pop     {r4, r6, r7, pc}          // a function return

I have a variable bar and here's its corresponding DWARF DIE:

 <2><3b>: Abbrev Number: 3 (DW_TAG_formal_parameter)
    <3c>   DW_AT_location    : 0x0 (location list)
    <40>   DW_AT_name        : (indirect string, offset: 0x9e): bar
    <44>   DW_AT_decl_file   : 1
    <45>   DW_AT_decl_line   : 34
    <46>   DW_AT_type        : <0x153>

 // Its location list
    00000000 00008124 0000812a (DW_OP_reg0 (r0))
    0000000b 0000812a 00008136 (DW_OP_reg4 (r4))
    00000016 <End of list>

As you can see, it says that we can find bar in r4 from 0x812a to 0x8134 (poplt).  However, this is only true when the cmp instruction at 0x812e yields less than (lt).  So if the value in r0 is greater than 1 (which is the case of my input), we should still be able to read the value of bar from r4 in the remaining of the function.

I don't know if we can consider this a bug, because I don't even know what should be the correct location information for bar. However, in this case, since the conditional instruction that clobbers r4 is a function return, I'd expect to read the value of bar from r4 in the remaining of the function.

I can't tell for sure whether the debug info is correct without also seeing the source code, but as a general point: Debug information is must-information that holds over all paths through the program. Debug information that is only accurate for some paths is a bug. A serious bug, because if the user can't rely on the debug info to be correct in some cases, they can't rely on any of the debug info to be correct.

-- adrian



If the conditional instruction poplt was addlt r4, r0, 3 for example, what should be the correct location list of bar?

For now, my only idea is to check if the clobbering MI is a conditional return in DbgValueHistoryCalculator which computes the end address of a location llist entry. But I do not feel like this is the correct fix though.

Looking forward to hearing your thoughts on this,

Thank you for reading this,

Son Tuan Vu

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