CVS commit: src/external/gpl3/gcc/dist/gcc/config/arm

["Taylor R Campbell" <riastradh%netbsd.org@localhost>]

Module Name:    src
Committed By:   riastradh
Date:           Sat Jul 25 19:01:48 UTC 2026

Modified Files:
        src/external/gpl3/gcc/dist/gcc/config/arm: arm.cc

Log Message:
gcc/arm: For -mtp=soft, ensure stack alignment even in leaves.

The option -mtp=soft, which is the default on earmv5, makes queries
to the thread pointer, for access to static (`initial-exec')
thread-local storage, go through the C runtime subroutine
__aeabi_read_tp.  (For earmv>=6, we use the cp15 register via a
single instruction.)

Thus procedures which gcc thinks of as leaf procedures that use
__aeabi_read_tp are not really leaf procedures -- and even though
__aeabi_read_tp itself doesn't use the stack pointer at all,
resolving the symbol may take a detour through the dynamic linker,
which does rely on an aligned stack pointer.

Without this change, code like

__thread int x = 0;
int *
get_x(void)
{
        return &x;
}

is compiled into:

00000000 <get_x>:
get_x():
   0:   e52de004        push    {lr}            @ (str lr, [sp, #-4]!)
   4:   ebfffffe        bl      0 <__aeabi_read_tp>
                        4: R_ARM_CALL   __aeabi_read_tp
   8:   e59f3004        ldr     r3, [pc, #4]    @ 14 <get_x+0x14>
   c:   e0830000        add     r0, r3, r0
  10:   e49df004        pop     {pc}            @ (ldr pc, [sp], #4)
  14:   00000000        .word   0x00000000
                        14: R_ARM_TLS_LE32      .LANCHOR0

With this change, extend the _existing_ push and pop instructions in
the prologue and epilogue to save and restore r4 arbitrarily (not
that the content is useful for anything; it just keeps the stack
pointer aligned to multiple of 8):

00000000 <get_x>:
get_x():
   0:   e92d4010        push    {r4, lr}
   4:   ebfffffe        bl      0 <__aeabi_read_tp>
                        4: R_ARM_CALL   __aeabi_read_tp
   8:   e59f3004        ldr     r3, [pc, #4]    @ 14 <get_x+0x14>
   c:   e0830000        add     r0, r3, r0
  10:   e8bd8010        pop     {r4, pc}
  14:   00000000        .word   0x00000000
                        14: R_ARM_TLS_LE32      .LANCHOR0

Ideally, we would ensure stack alignment only for `leaf' procedures
that actually use __aeabi_read_tp.  But I don't know how to query
that in this context.  And in any case, this change only applies to
subroutines that are already saving 2n+1 registers for some n,
picking some arbitrary register to round it up to 2n+2.  For example:

int y;
int *
get_y(void)
{
        return &y;
}
int *
get_y_with_r4(void)
{
        asm volatile("" ::: "r4");
        return &y;
}

With this change, we get:

00000018 <get_y>:
get_y():
  18:   e59f0000        ldr     r0, [pc]        @ 20 <get_y+0x8>
  1c:   e12fff1e        bx      lr
  20:   00000000        .word   0x00000000
                        20: R_ARM_ABS32 .bss

00000024 <get_y_with_r4>:
get_y_with_r4():
  24:   e92d0030        push    {r4, r5}
  28:   e8bd0030        pop     {r4, r5}
  2c:   e59f0000        ldr     r0, [pc]        @ 34 <get_y_with_r4+0x10>
  30:   e12fff1e        bx      lr
  34:   00000000        .word   0x00000000
                        34: R_ARM_ABS32 .bss

In contrast, with earmv>=6, get_y would be unchanged, but
get_y_with_r4 would be:

00000020 <get_y_with_r4>:
get_y_with_r4():
  20:   e52d4004        push    {r4}            @ (str r4, [sp, #-4]!)
  24:   e59f0004        ldr     r0, [pc, #4]    @ 30 <get_y_with_r4+0x10>
  28:   e49d4004        pop     {r4}            @ (ldr r4, [sp], #4)
  2c:   e12fff1e        bx      lr
  30:   00000000        .word   0x00000000
                        30: R_ARM_ABS32 .bss

Note that the number of instructions hasn't changed; all that has
changed is whether we save and restore one or two registers in the
function prologue and epilogue (or more generally, 2n+1 vs 2n+2
registers).

PR lib/57638: thread local storage broken on evbarm (armv5)

Upstream bug: https://gcc.gnu.org/bugzilla/show_bug.cgi?id=126394

ok skrll@


To generate a diff of this commit:
cvs rdiff -u -r1.1.1.4 -r1.2 src/external/gpl3/gcc/dist/gcc/config/arm/arm.cc

Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.