msp430-elf-as reloc errors

Hi David,

This is a known problem/feature with the assembler. At stake here is the problem of using the value of a symbol and redefining it in the same file. It used to work because the value(s) of the symbol were computed by the assembler and put into the object file directly.
This no longer happens because the MSP430 assembler now delays computing the value of symbols (in the code sections) until link time. This is necessary because linker relaxation might change the number or size of instructions in a code section, so the position of the symbols inside the section can change too.

So the assembler tells the linker to install the symbol's values once the code has been relaxed, and to do this it uses something called relocs (or relocations). Each reloc contains an address where the change takes place, a code to tell the linker what kind of value it is supposed to compute and a symbol to be used in computing the value. But this only works if the symbol has a single value. If the symbol's value keeps on changing, then it cannot be used in a reloc, and hence the linker cannot be told how to install the correct values.

This also explains why this problem does not arise on targets that do not perform linker relaxation, eg the ARM.

All of which does not help you. There are a couple of possible workarounds however:

1. Use local labels instead of local symbols, like this:

.macro CODE lex, name, label
.word 1b
1:
.byte \lex
.ascii "\name"
.align 1
\label :
.endm

1:

This should work provided that the 1: local label is not used elsewhere in the assembler source. (A different value for the local label can be chosen if this is the case).



2. Place the table in a data section, not a code section, like this:

.data

.macro CODE lex, name, label
.word _link
.equ _link, .
.byte \lex
.ascii "\name"
.align 1
\label :
.endm

The restriction on not redefining symbols does not apply inside data sections.

I hope that this helps.

Cheers
Nick

Hi David,

Re: using local labels. Can you not just put a .word 0 at the first label ? Ie:

.text
1:
.word 0

.macro CODE lex, name, label
.word 1b
1:
.byte \lex
.ascii "\name"
.align 1
\label :
.endm

Re: using the macro in a data section - I do not understand how code is ending up in RAM. Are you switching back to the .text section after using the macro ? Or maybe a version like this would be better:

.macro CODE lex, name, label
.pushsection .data
.word _link
.equ _link, .
.byte \lex
.ascii "\name"
.align 1
\label :
.popsection
.endm

As for the documentation, it is wrong, (and yes I will be updating it). It is now the case that for some targets, MSP430 included, you cannot redefine symbols after they have been used.

A --norelax option is a possibility although it will in general tend to result in bigger overall executables. Linker relaxation really is quite useful for the MSP430.

Cheers
Nick

Hi David,

> msp430-elf-as should still have a way to enable or disable relaxation errors.

Please could you try the patch below and let me know if it does what you want. The patch has two parts: 1) it extends the -Q option of msp430-elf-as so that it also disables assembler support for linker relaxation as well. 2) It updates the documentation describing the .set pseudo-op to mention that on some targets symbols cannot be redefined if the expression involves symbolic values.

Cheers
Nick

diff --git a/gas/config/tc-msp430.c b/gas/config/tc-msp430.c
index 2891c13..dced7bf 100644
--- a/gas/config/tc-msp430.c
+++ b/gas/config/tc-msp430.c
@@ -798,6 +798,7 @@ md_parse_option (int c, char * arg)

case OPTION_RELAX:
msp430_enable_relax = 1;
+ linkrelax = 0;
return 1;

case OPTION_POLYMORPHS:
@@ -3889,6 +3890,8 @@ msp430_fix_adjustable (struct fix *fixp ATTRIBUTE_UNUSED)
&& ((S_GET_SEGMENT (fixp->fx_addsy)->flags & SEC_CODE) == 0))
return TRUE;

+ if (! linkrelax && fixp->fx_addsy)
+ return TRUE;
return FALSE;
}

@@ -3929,6 +3932,9 @@ msp430_allow_local_subtract (expressionS * left,
if (left->X_add_symbol == right->X_add_symbol)
return TRUE;

+ if (! linkrelax)
+ return TRUE;
+
/* We have to assume that there may be instructions between the
two symbols and that relaxation may increase the distance between
them. */
diff --git a/gas/config/tc-msp430.h b/gas/config/tc-msp430.h
index 08115ac..d506f5f 100644
--- a/gas/config/tc-msp430.h
+++ b/gas/config/tc-msp430.h
@@ -158,10 +158,10 @@ extern bfd_boolean msp430_allow_local_subtract (expressionS *, expressionS *, se
because it breaks linker relaxations. This could be fixed in the
linker, but this fix is simpler, and it pretty much only affects
object size a little bit. */
-#define TC_FORCE_RELOCATION_SUB_SAME(FIX, SEC) \
- ( ((SEC)->flags & SEC_CODE) != 0 \
- || ((SEC)->flags & SEC_DEBUGGING) != 0 \
- || ! SEG_NORMAL (SEC) \
+#define TC_FORCE_RELOCATION_SUB_SAME(FIX, SEC) \
+ ((linkrelax && (((SEC)->flags & SEC_CODE) != 0 \
+ || ((SEC)->flags & SEC_DEBUGGING) != 0)) \
+ || ! SEG_NORMAL (SEC) \
|| TC_FORCE_RELOCATION (FIX))

/* We validate subtract arguments within tc_gen_reloc(),
@@ -170,4 +170,4 @@ extern bfd_boolean msp430_allow_local_subtract (expressionS *, expressionS *, se

#define DWARF2_USE_FIXED_ADVANCE_PC 1

-#define TC_LINKRELAX_FIXUP(seg) ((seg->flags & SEC_CODE) || (seg->flags & SEC_DEBUGGING))
+#define TC_LINKRELAX_FIXUP(seg) (linkrelax && ((seg->flags & SEC_CODE) || (seg->flags & SEC_DEBUGGING)))
diff --git a/gas/doc/as.texinfo b/gas/doc/as.texinfo
index 1c2fe08..cc78449 100644
--- a/gas/doc/as.texinfo
+++ b/gas/doc/as.texinfo
@@ -6351,7 +6351,14 @@ changes @var{symbol}'s value and type to conform to
@var{expression}. If @var{symbol} was flagged as external, it remains
flagged (@pxref{Symbol Attributes}).

-You may @code{.set} a symbol many times in the same assembly.
+You may @code{.set} a symbol many times in the same assembly provided that the
+values given to the symbol are constants. Values that are based on expressions
+involving other symbols are allowed, but some targets may restrict this to only
+being done once per assembly. This is because those targets do not set the
+addresses of symbols at assembly time, but rather delay the assignment until a
+final link is performed. This allows the linker a chance to change the code in
+the files, changing the location of, and the relative distance between, various
+different symbols.

If you @code{.set} a global symbol, the value stored in the object
file is the last value stored into it.

Hi David,

> Would this be the "-mQ - enable relaxation at assembly time.DANGEROUS!" flag that you are changing?

Yes.

> If so, what is dangerous about it?

I don't know. :-(  That code was written a long time ago, and not by me, so I am not sure what it means.  I *think* that it means that it is dangerous to use this option when assembling code generated by the compiler, as the compiler relies upon the linker relaxations to make things work.  But that is just a guess.

> 2) It has been a while since I have built the GCC suite from source but I gave it a whirl.

You don't actually need to build all of gcc.  You just need to built GAS, which should be a lot simpler.  (GAS is part of the binutils package, and it can be  built independently of gcc).  So in theory all you need to do is:

$ tar --bzip2 -xf binutils-2.25.tar.bz2
$ cd binutils-2.25

$ patch -i experimenal-msp430-as.patch
$ ./configure --target=msp430 --prefix=/usr/local/msp430
$ make all-gas

$ ./gas/as-new my-test-file.s

> In file included from size.c:33:0:

> sysdep.h:30:20: fatal error: bfdver.h: No such file or directory #include "bfdver.h"

  Hmm, strange.  bfdver.h is a constructed header file that should be found in the bfd/ directory of your build tree.  Possibly something went wrong when the bfd library was being built ?

Cheers

  Nick