https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97754
Bug ID: 97754
Summary: arm/lib1funcs.S (RETLDM): CFI may be incorrect
Product: gcc
Version: 11.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: libgcc
Assignee: unassigned at gcc dot gnu.org
Reporter: martingalvan at sourceware dot org
Target Milestone: ---
Hi all, I'm looking at the RETLDM macro for the ARM libgcc code:
.macro RETLDM regs=, cond=, unwind=, dirn=ia
#if defined (__INTERWORKING__)
.ifc "\regs",""
ldr\cond lr, [sp], #8
.else
# if defined(__thumb2__)
pop\cond {\regs, lr}
# else
ldm\cond\dirn sp!, {\regs, lr}
# endif
.endif
.ifnc "\unwind", ""
/* Mark LR as restored. */
97: cfi_pop 97b - \unwind, 0xe, 0x0
.endif
bx\cond lr
#else
<...>
.endm
Here we can see that for the __INTERWORKING__ case, some CFI data is emitted if
we have an 'unwind' argument. However, I believe this may be wrong if a RETLDM
macro appears in the middle of another function. For the __INTERWORKING__ case,
the resulting output would be e.g:
pop {r4, r5, lr}
cfi_pop <...>
bx lr
<rest of the function>
Here, 'cfi_pop' would affect the rest of the function unless the caller is
careful enough to restore the CFI status for lr after doing RETLDM. This can
happen e.g. if we pass a 'cond' argument, which means the branch may not be
taken.
Unless I'm mistaken, the way to fix this would be to introduce a couple of
.cfi_remember/restore_state directives:
.ifnc "\unwind", ""
/* Mark LR as restored. */
.cfi_remember_state
97: cfi_pop 97b - \unwind, 0xe, 0x0
.endif
bx\cond lr
.ifnc "\unwind", ""
.cfi_restore_state
.endif
If this is correct, I'll send in a patch to add this change.
Speaking of this file, I have a couple more questions:
1. The file is defining macros like cfi_start, cfi_push, etc which insert the
CFI data directly in binary form. Why can't we use gas' .cfi_* directives as we
do elsewhere? If we can, I'll send a separate patch to add this change as well.
2. Why is the __INTERWORKING__ macro only defined for ARMv4t? I'm not saying
that this is wrong, just curious.
Thanks.
