Re: [RFC PATCH v3 11/12] powerpc: Remove unreachable() from WARN_ON()

Le 27/06/2022 à 17:35, Sathvika Vasireddy a écrit :

On 25/06/22 12:16, Christophe Leroy wrote:

quoted

Le 24/06/2022 à 20:32, Sathvika Vasireddy a écrit :

quoted

objtool is throwing *unannotated intra-function call*
warnings with a few instructions that are marked
unreachable. Remove unreachable() from WARN_ON()
to fix these warnings, as the codegen remains same
with and without unreachable() in WARN_ON().

Did you try the two exemples described in commit 1e688dd2a3d6
("powerpc/bug: Provide better flexibility to WARN_ON/__WARN_FLAGS() with
asm goto") ?

Without your patch:

00000640 <test>:
   640:    81 23 00 84     lwz     r9,132(r3)
   644:    71 29 40 00     andi.   r9,r9,16384
   648:    40 82 00 0c     bne     654 <test+0x14>
   64c:    80 63 00 0c     lwz     r3,12(r3)
   650:    4e 80 00 20     blr
   654:    0f e0 00 00     twui    r0,0

00000658 <test9w>:
   658:    2c 04 00 00     cmpwi   r4,0
   65c:    41 82 00 0c     beq     668 <test9w+0x10>
   660:    7c 63 23 96     divwu   r3,r3,r4
   664:    4e 80 00 20     blr
   668:    0f e0 00 00     twui    r0,0
   66c:    38 60 00 00     li      r3,0
   670:    4e 80 00 20     blr


With your patch:

00000640 <test>:
   640:    81 23 00 84     lwz     r9,132(r3)
   644:    71 29 40 00     andi.   r9,r9,16384
   648:    40 82 00 0c     bne     654 <test+0x14>
   64c:    80 63 00 0c     lwz     r3,12(r3)
   650:    4e 80 00 20     blr
   654:    0f e0 00 00     twui    r0,0
   658:    4b ff ff f4     b       64c <test+0xc>        <==

0000065c <test9w>:
   65c:    2c 04 00 00     cmpwi   r4,0
   660:    41 82 00 0c     beq     66c <test9w+0x10>
   664:    7c 63 23 96     divwu   r3,r3,r4
   668:    4e 80 00 20     blr
   66c:    0f e0 00 00     twui    r0,0
   670:    38 60 00 00     li      r3,0            <==
   674:    4e 80 00 20     blr                <==
   678:    38 60 00 00     li      r3,0
   67c:    4e 80 00 20     blr

The builtin variant of unreachable (__builtin_unreachable()) works.

How about using that instead of unreachable() ?

That seems odd.

Look at linux/compiler.h

It seems like unreachable() exists to help objtool.

Christophe