On Mon, 11 Jul 2022 13:57:40 +0100 Andre Przywara andre.przywara@arm.com wrote:
Hi,
On Sun, 10 Jul 2022 03:09:53 -0400 Jesse Taube mr.bossman075@gmail.com wrote:
Hi Jesse,
In Binutils 2.37 the ADR instruction has changed use alternate instructions.
Can you elaborate on this? What has changed exactly, and why? Looking at the commit you mention below I don't see an immediate problem that would require code changes? Also it speaks of forward references, but this one is not one? And I didn't spot any difference between 2.38 and 2.35, at least not in my isolated test (but I didn't bother to compile a whole stage 1 GCC with newer binutils yet).
OK, so digging a bit deeper I think I have an idea: With as 2.35 I get: 080007cc <relocate_code>: 80007cc: f2af 0304 subw r3, pc, #4
whereas with 2.38 it's: 080007cc <relocate_code>: 80007cc: f2af 0303 subw r3, pc, #3
the latter looks correct since we compile relocate.S with -mthumb -mthumb-interwork, so the lowest bit of the *function* address should be set, to indicate this is a Thumb function. And "ENTRY(relocate_code)" clearly tells the assembler that relocate_code is a function entry point, so should carry the instruction set flag in bit 0. However we don't use the result of "adr" as an argument for a bx call later, but to calculate some relocation offset, so the bit is getting in the way. Without thinking too much about this, wouldn't it help to just always clear bit 0 in r3? Or probably better: to have an additional label, which is not marked as a function entry point?
Cheers, Andre
The change causes armv7-m to not boot.
What does "causes armv7-m to not boot" mean? It compiles fine, but hangs or crashes? Can you show the relevant disassembly from both binutils versions?
And from trying to reproduce this minimally, do we need a ".syntax unified" in the .S file?
Signed-off-by: Jesse Taube Mr.Bossman075@gmail.com
arch/arm/lib/relocate.S | 8 +++++++- 1 file changed, 7 insertions(+), 1 deletion(-)
diff --git a/arch/arm/lib/relocate.S b/arch/arm/lib/relocate.S index 14b7f61c1a..22c419534f 100644 --- a/arch/arm/lib/relocate.S +++ b/arch/arm/lib/relocate.S @@ -78,7 +78,13 @@ ENDPROC(relocate_vectors) */
ENTRY(relocate_code)
- adr r3, relocate_code
+/*
- Binutils doesn't comply with the arm docs for adr in thumb2
- from commit d3e52e120b68bf19552743fbc078e0a759f48cb7 onward
- to remove ambiguity explicitly define the pseudo-instruction
- */
- mov r3, pc
- subs r3, #4
But this will break ARM, won't it? Because it would require to subtract #8? I mean there is a reason for this adr instruction, because this offset calculation is best left to the assembler. Not to speak of the fragility of assuming that the relocate_code label is pointing to the very first instruction. The ENTRY macro could also insert instructions.
Cheers, Andre
ldr r1, _image_copy_start_ofs add r1, r3 /* r1 <- Run &__image_copy_start */ subs r4, r0, r1 /* r4 <- Run to copy offset */