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Save and restore r9 register in arm ubldr. In old gcc 4.2, r9 was a callee-

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saved register, but in arm EABI it may be either callee-saved or dedicated
to some special purpose (such as a TLS pointer).  It appears clang does not
treat it as a callee-saved register (instead using it as another work
register, similar to r12).

Another important side effect of these changes is that saving an extra
register in the push/pop statements keeps the stack aligned to an 8-byte
boundary during the self_reloc() call, as it always should have been.

As stated in the PR...

Essentially the important caller-saved registers are pushed (r0, r1, r9, lr)
before the relocation call, and popped after.  Then r8/r9 are saved as usual
for the syscall trampoline, and lr is stored in r8 (now free) as a
callee-saved value before calling into `main`.

The call to `main` can no longer be a tail call because we must restore r9
especially after main returns (although since we have used r8 to hold lr we
must also restore this).

PR:		224008
This commit is contained in:
Ian Lepore 2017-12-10 21:51:27 +00:00
parent 3102bbe940
commit 12b92a343c
1 changed files with 19 additions and 13 deletions
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32
stand/arm/uboot/start.S
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@ -46,11 +46,8 @@ _start:
mcr p15, 0, ip, c1, c0, 0
#endif
/*
* Save r0 and r1 (argc and argv passed from u-boot), and lr (trashed
* by the call to self_reloc below) until we're ready to call main().
*/
push {r0, r1, lr}
/* Save the arguments and return register before calling self_reloc */
push {r0, r1, r9, lr}
/*
* Do self-relocation when the weak external symbol _DYNAMIC is non-NULL.
@ -68,22 +65,31 @@ _start:
addne r1, r1, r0 /* r1 = dynamic section physaddr. */
blne _C_LABEL(self_reloc) /* Do reloc if _DYNAMIC is non-NULL. */
/* Restore saved arguments */
pop {r0, r1, r9, lr}
/* Hint where to look for the API signature */
ldr ip, =uboot_address
str sp, [ip]
/* Save U-Boot's r8 and r9 */
/* Save U-Boot's r8 and r9 for syscall trampoline */
ldr ip, =saved_regs
str r8, [ip, #0]
str r9, [ip, #4]
str r8, [ip, #0] /* old gd pointer (use to hold lr) */
str r9, [ip, #4] /* new gd pointer */
/*
* First restore argc, argv, and the u-boot return address, then
* Start loader. This is basically a tail-recursion call; if main()
* returns, it returns to u-boot (which reports the value returned r0).
* Start loader. Save return address first (r8 is available from
* trampoline save).
*/
pop {r0, r1, lr}
b main
mov r8, lr
bl main
mov lr, r8
/* Restore U-Boot environment */
ldr ip, =saved_regs
ldr r8, [ip, #0]
ldr r9, [ip, #4]
mov pc, lr
/*
* Data for self-relocation, in the text segment for pc-rel access.