On Mon, 11 Jul 2022 13:57:40 +0100 Andre Przywara <andre.przyw...@arm.com> wrote:
Hi, > On Sun, 10 Jul 2022 03:09:53 -0400 > Jesse Taube <mr.bossman...@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.bossman...@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 */ >
