https://sourceware.org/bugzilla/show_bug.cgi?id=20989
--- Comment #2 from Rainer Orth <ro at CeBiTec dot Uni-Bielefeld.DE> ---
> --- Comment #1 from Alan Modra <amodra at gmail dot com> ---
> How is crt1.o testing that __start_crt_compiler is undefined? I'm talking
> about an "if (foo)" test used in a call to a weak function:
> if (foo)
> foo ();
You've got this (I'm attaching both the crt1.o and empty-ld and
empty-gld executables for reference) in crt1.o:
crt1.o:
cc __start_crt+0x4c: 2f 00 00 00 sethi %hi(0x0), %l7
d0 __start_crt+0x50: f4 74 60 00 stx %i2, [%l1]
d4 __start_crt+0x54: ac 1d e0 00 xor %l7, 0x0, %l6
d8 __start_crt+0x58: ea 5c 00 16 ldx [%l0 + %l6], %l5
dc __start_crt+0x5c: 02 c5 40 09 brz,pn %l5, +0x24
<__start_crt+0x80>
^ offsets (in hex) from the beginning of .text to allow matching with
the relocs below.
This is the
if (__start_crt_compiler)
__start_crt_compiler ();
part ...
e0 __start_crt+0x60: 91 3e 20 00 sra %i0, 0x0, %o0
e4 __start_crt+0x64: 40 00 00 00 call +0x0
<__start_crt+0x64>
... and here are the corresponding relocs.
[17] R_SPARC_GOTDATA_OP_HIX22 0xcc 0 0 .text
__start_crt_compiler
[18] R_SPARC_GOTDATA_OP_LOX10 0xd4 0 0 .text
__start_crt_compiler
[19] R_SPARC_GOTDATA_OP 0xd8 0 0 .text
__start_crt_compiler
[20] R_SPARC_WPLT30 0xe4 0 0 .text
__start_crt_compiler
When single-stepping through this part of empty-ld (ld output), I find:
__start_crt+0x4c: 2f 00 00 00 sethi %hi(0x0), %l7
__start_crt+0x50: f4 74 60 00 stx %i2, [%l1]
__start_crt+0x54: ac 1d e0 08 xor %l7, 0x8, %l6
__start_crt+0x58: ea 5c 00 16 ldx [%l0 + %l6], %l5
__start_crt+0x5c: 02 c5 40 09 brz,pn %l5, +0x24
<__start_crt+0x80>
__start_crt+0x60: 91 3e 20 00 sra %i0, 0x0, %o0
__start_crt+0x64: 40 00 00 00 call +0x0
<__start_crt+0x64>
__start_crt+0x48: brnz,pn %l2, 0x100000b94 <__start_crt+0x54>
(gdb) p/x $l2
$5 = 0xffffffff7ffffb08
__start_crt+0x4c: sethi %hi(0), %l7
__start_crt+0x54: xor %l7, 8, %l6
__start_crt+0x58: ldx [ %l0 + %l6 ], %l5
__start_crt+0x5c: brz,pn %l5, 0x100000bc0 <__start_crt+0x80>
(gdb) p/x $l5
$4 = 0x0
I.e. __start_crt_compiler is 0 and the branch is taken, skipping the
call.
__start_crt+0x60: sra %i0, 0, %o0
__start_crt+0x80: sethi %hi(0x100000), %i3
On the other hand, with the gld-produced executable, I get
__start_crt+0x4c: 2f 00 04 02 sethi %hi(0x100800), %l7
__start_crt+0x50: f4 74 60 00 stx %i2, [%l1]
__start_crt+0x54: ac 1d fd f0 xor %l7, -0x210, %l6
__start_crt+0x58: aa 04 00 16 add %l0, %l6, %l5
__start_crt+0x5c: 02 c5 40 09 brz,pn %l5, +0x24
<__start_crt+0x80>
__start_crt+0x60: 91 3e 20 00 sra %i0, 0x0, %o0
__start_crt+0x64: 7f ff fe 57 call -0x6a4 <0x100000000>
__start_crt+0x48: brnz,pn %l2, 0x100000694 <__start_crt+0x54>
(gdb) p/x $l2
$4 = 0xffffffff7ffff9d8
__start_crt+0x4c: sethi %hi(0x100800), %l7
__start_crt+0x54: xor %l7, -528, %l6
__start_crt+0x58: add %l0, %l6, %l5
__start_crt+0x5c: brz,pn %l5, 0x1000006c0 <__start_crt+0x80>
(gdb) p/x $l5
$6 = 0x100000000
__start_crt+0x60: sra %i0, 0, %o0
__start_crt+0x64: call 0x100000000
I.e. __start_crt_compiler is non-NULL, the call is taken, but the
destination is invalid, leading to the SIGILL.
Rainer
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