[PATCH 4.0/4 v3] MIPS/GAS: Propagate symbol attributes

["Maciej W. Rozycki" <macro at codesourcery dot com>]

Hi Richard,

On Sat, 30 Oct 2010, Richard Sandiford wrote:

> >  There are several cases where ELF symbol attributes are not correctly set 
> > for symbols leading to all kinds of odd side effects for MIPS16 code (and 
> > with the upcoming change also for microMIPS code).
> >
> >  For example this program:
> >
> > 	.text
> > foo:
> > 	xor	$16, $17
> > 	.set	fnord, . + 2
> > 	addu	$2, $3, $4
> > 	xor	$5, $6
> > bar:
> > 	subu	$7, $16
> >
> > assembles to this:
> >
> > Disassembly of section .text:
> >
> > 00000000 <foo>:
> >    0:	e82e     	xor	s0,s1
> >    2:	e389     	addu	v0,v1,a0
> >
> > 00000004 <fnord>:
> >    4:	edcee71f 	swc3	$14,-6369(t6)
> >
> > 00000006 <bar>:
> >    6:	e71f     	subu	a3,s0
> >
> > Notice how fnord's ELF attributes have not been set to indicate a MIPS16 
> > symbol and the resulting confusion (this symbol, of course, if used as a 
> > jump target will not set the ISA bit correctly).  Similar symptoms are 
> > seen with equated symbols (defined with .eqv) although the attributes are 
> > lost at a different stage of assembly.
> >
> >  Here's a fix for these problems and a test case covering hopefully most 
> > of them as well as those fixed by patches submitted previously in this series.
> >
> >  There are two new functions defined:
> >
> > - mips_elf_copy_symbol_attributes() -- used for symbols defined with .set.  
> >   These, if calculated from a label that has been defined for the current 
> >   location (the "dot" special symbol being a prominent example, but any 
> >   label will actually do) will not have their ELF attributes set, because 
> >   the original label will only get them set once an instruction has been 
> >   emitted.  A solution is to place the newly defined symbol on the list of 
> >   labels too.
> >
> > - mips_elf_propagate_symbol_attributes() -- used for symbols defined with 
> >   .eqv.  They are processed late and currently attributes are copied only 
> >   for exact copies of other symbols, i.e.:
> >
> > 	.eqv	foo, bar
> >
> >   but not:
> >
> > 	.eqv	foo, bar + 2
> >
> >   In the case of MIPS16 (and microMIPS) ELF attribute we want it to be 
> >   propagated even for offsetted symbols, hence the new hook in 
> >   resolve_symbol_value().
> >
> >  Finally I realised the uniqueness of the -mmips:16 option to `objdump' 
> > that causes all the disassembled to be treated as MIPS16 code, regardless 
> > of symbol annotations found.  This actually covers the bugs addressed 
> > here, hence I'm removing it.  The implication is all MIPS16 tests need to 
> > have a label at the beginning to set the ISA mode of the disassembler 
> > correctly.
> >
> >  Perhaps the behaviour of the -mmips:16 option should be changed instead 
> > so that it respects symbol annotations.  I think it would make sense -- 
> > the option should normally only be needed for binary objects with no 
> > sufficient symbol information.  OTOH, the current behaviour is good in 
> > case `objdump' gets confused because of a bug or a broken binary, hence I 
> > have no strong preference actually towards making the change.
> >
> >  If we agree to make it after all, then the option can be added back.  I 
> > believe -mmips:micromips behaves the same.  Additionally odd text symbols 
> > with no ELF annotation are unconditionally treated as MIPS16 code.  This 
> > should probably be changed.

 As a side note I have actually fixed this oddity with the lastest 
microMIPS patch -- odd text symbols with no ELF annotation are treated as 
either MIPS16 or microMIPS code based on the ELF file header flags; if no 
MIPS16 or microMIPS flag is set, then the MIPS16 ASE is assumed preserving 
the current behaviour.

> >  Compared to the original version, this change has only been trivially 
> > updated to take the new treatment of symbols equated to an expression 
> > involving "." into account.  The update is limited to the test case 
> > according to the comment I'll just repeat here: "Move the location counter 
> > away from the end of code to avoid the final values of symbols equated to 
> > expressions involving the counter interfering with disassembly."  Without 
> > this change MIPS16 code would be disassembled incorrectly as the equated 
> > symbols have the MIPS16 attribute clear (quite correctly, because the 
> > final value of "." does not point to MIPS16 code).  Dumps have been 
> > adjusted accordingly.
> 
> Sorry for not really getting to this patch when you posted the original
> series.  Do you actually have a "real world" use case for this though?
> Why wouldn't you just put "fnord:" in the appropriate place?

 Please note this issue only affects disassembly -- I have not observed 
any other unexpected symptoms.  In particular all the relocated entities 
referring to the symbols concerned get the ISA mode set regardless of this 
change.

 So it's not about whether we should treat these symbols as MIPS16 code
references or not, but to match the reality and not mislead the user into 
thinking standard MIPS code is being concerned.

> I'm not certain that, in general, we can say that "mips16 symbol + offset"
> should always be a mips16 symbol.

 Well, TBH I think it should be doing so by definition -- if you offset a 
MIPS16 symbol (i.e. a text symbol with ".set mips16" in effect), then you 
get a MIPS16 symbol (i.e. another text symbol carrying the ".set mips16" 
property).  If you don't want to get a MIPS16 symbol, then start with a 
non-MIPS16 symbol in the first place.

 For example this piece of code does the right thing (which may be good to 
know to some in case a piece of MIPS16 code has to be referred to as data 
for run-time relocation or something):

$ cat mips16.s
	.text
	.set	mips16
	.globl	foo
	.globl	bar
foo:
	.fill	0
bar:
	jr	$31

	.data
	.word	foo
	.word	bar
$ mips-sde-elf-as -o mips16.o mips16.s
$ mips-sde-elf-ld -ebar -o mips16 mips16.o
$ mips-sde-elf-objdump -s -j .data mips16

mips16:     file format elf32-tradbigmips

Contents of section .data:
 410078 00400074 00400075                    .@.t.@.u        

[As it is `objdump -d' doesn't get the disassembly of bar() right, but I 
believe it's fixed as a side effect of the fix made with the microMIPS 
change side-noted above.]  You can then offset "foo" or "bar" further as 
required and get the desired result.

 I have updated the change to include documentation in internals.texi and 
regenerated it against the current shape of the code patched.  No other 
changes compared to the original and it still passes regression tests for 
mips-sde-elf and mips-linux-gnu.

2010-12-08  Maciej W. Rozycki  <macro@codesourcery.com>

	gas/
	* config/tc-mips.c (insn_label_recording): New variable.
	(md_begin): Set insn_label_recording.
	(md_mips_end): Clear insn_label_recording.
	(mips_elf_copy_symbol_attributes): New function.
	(mips_elf_propagate_symbol_attributes): New function.
	* config/tc-mips.h (TC_COPY_SYMBOL_ATTRIBUTES): New macro.
	(TC_PROPAGATE_SYMBOL_ATTRIBUTES): Likewise.
	(mips_elf_copy_symbol_attributes): New prototype.
	(mips_elf_propagate_symbol_attributes): Likewise.
	* symbols.c (resolve_symbol_value): Call
	TC_PROPAGATE_SYMBOL_ATTRIBUTES for symbols that are not exact
	copies of originals.
	* doc/internals.texi (CPU backend): Document 
	TC_PROPAGATE_SYMBOL_ATTRIBUTES.

	gas/testsuite/
	* gas/mips/branch-self.d: New test for various definitions of
	labels.
	* gas/mips/branch-self.s: Source for the new test.
	* gas/mips/mips.exp: Run the new test.  Remove -mmips:16 from
	"mips16" architecture.

  Maciej

binutils-gas-propagate-attrs.diff
Index: binutils-fsf-trunk-quilt/gas/config/tc-mips.c
===================================================================
--- binutils-fsf-trunk-quilt.orig/gas/config/tc-mips.c	2010-12-08 00:26:16.000000000 +0000
+++ binutils-fsf-trunk-quilt/gas/config/tc-mips.c	2010-12-08 00:26:16.000000000 +0000
@@ -1265,6 +1265,8 @@ struct insn_label_list
 static struct insn_label_list *free_insn_labels;
 #define label_list tc_segment_info_data.labels
 
+static int insn_label_recording;
+
 static void mips_clear_insn_labels (void);
 
 static inline void
@@ -2104,11 +2106,15 @@ md_begin (void)
 
   if (mips_fix_vr4120)
     init_vr4120_conflicts ();
+
+  insn_label_recording = 1;
 }
 
 void
 md_mips_end (void)
 {
+  insn_label_recording = 0;
+
   if (! ECOFF_DEBUGGING)
     md_obj_end ();
 }
@@ -14728,9 +14734,38 @@ mips_define_label (symbolS *sym)
   dwarf2_emit_label (sym);
 #endif
 }
-
+
 #if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
 
+/* Record copies of instruction labels made, for later MIPS16 symbol
+   annotation.  */
+
+void
+mips_elf_copy_symbol_attributes (symbolS *dest, symbolS *src ATTRIBUTE_UNUSED)
+{
+  if (!IS_ELF || !insn_label_recording)
+    return;
+
+  if (S_GET_SEGMENT (src) == now_seg
+      && symbol_get_frag (src) == frag_now
+      && symbol_constant_p (src)
+      && S_GET_VALUE (src) == frag_now_fix ()
+      && symbol_constant_p (dest))
+    mips_record_label (dest);
+}
+
+/* Propagate MIPS16 symbol annotation.  */
+
+void
+mips_elf_propagate_symbol_attributes (symbolS *dest, symbolS *src)
+{
+  if (!IS_ELF)
+    return;
+
+  if (ELF_ST_IS_MIPS16 (S_GET_OTHER (src)))
+    S_SET_OTHER (dest, ELF_ST_SET_MIPS16 (S_GET_OTHER (dest)));
+}
+
 /* Some special processing for a MIPS ELF file.  */
 
 void
Index: binutils-fsf-trunk-quilt/gas/config/tc-mips.h
===================================================================
--- binutils-fsf-trunk-quilt.orig/gas/config/tc-mips.h	2010-12-08 00:26:16.000000000 +0000
+++ binutils-fsf-trunk-quilt/gas/config/tc-mips.h	2010-12-08 00:26:16.000000000 +0000
@@ -124,6 +124,14 @@ extern void mips_frob_file (void);
 #if defined (OBJ_ELF) || defined (OBJ_MAYBE_ELF)
 #define tc_frob_file_after_relocs mips_frob_file_after_relocs
 extern void mips_frob_file_after_relocs (void);
+
+#define TC_COPY_SYMBOL_ATTRIBUTES(dest, src) \
+  mips_elf_copy_symbol_attributes (dest, src)
+extern void mips_elf_copy_symbol_attributes (symbolS *, symbolS *);
+
+#define TC_PROPAGATE_SYMBOL_ATTRIBUTES(dest, src) \
+  mips_elf_propagate_symbol_attributes (dest, src)
+extern void mips_elf_propagate_symbol_attributes (symbolS *, symbolS *);
 #endif
 
 #define tc_fix_adjustable(fixp) mips_fix_adjustable (fixp)
Index: binutils-fsf-trunk-quilt/gas/symbols.c
===================================================================
--- binutils-fsf-trunk-quilt.orig/gas/symbols.c	2010-12-08 00:26:16.000000000 +0000
+++ binutils-fsf-trunk-quilt/gas/symbols.c	2010-12-08 00:26:16.000000000 +0000
@@ -1184,12 +1184,19 @@ resolve_symbol_value (symbolS *symp)
 	      break;
 	    }
 
-	  if (finalize_syms && final_val == 0)
+	  if (finalize_syms)
 	    {
-	      if (LOCAL_SYMBOL_CHECK (add_symbol))
-		add_symbol = local_symbol_convert ((struct local_symbol *)
-						   add_symbol);
-	      copy_symbol_attributes (symp, add_symbol);
+	      if (final_val == 0)
+		{
+		  if (LOCAL_SYMBOL_CHECK (add_symbol))
+		    add_symbol = local_symbol_convert ((struct local_symbol *)
+						       add_symbol);
+		  copy_symbol_attributes (symp, add_symbol);
+		}
+#ifdef TC_PROPAGATE_SYMBOL_ATTRIBUTES
+	      else
+		TC_PROPAGATE_SYMBOL_ATTRIBUTES (symp, add_symbol);
+#endif
 	    }
 
 	  /* If we have equated this symbol to an undefined or common
Index: binutils-fsf-trunk-quilt/gas/testsuite/gas/mips/branch-self.d
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ binutils-fsf-trunk-quilt/gas/testsuite/gas/mips/branch-self.d	2010-12-08 00:26:16.000000000 +0000
@@ -0,0 +1,30 @@
+#objdump: -dr --prefix-addresses --show-raw-insn
+#name: MIPS branches to self
+#as: -32
+#source: branch-self.s
+
+# Test various ways to request a branch to self.
+
+.*: +file format .*mips.*
+
+Disassembly of section \.text:
+([0-9a-f]+) <[^>]*> ac620000 	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 1000ffff 	b	\1 <.*>
+([0-9a-f]+) <[^>]*> 00000000 	nop
+([0-9a-f]+) <[^>]*> ac620000 	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 1000ffff 	b	\1 <.*>
+([0-9a-f]+) <[^>]*> 00000000 	nop
+([0-9a-f]+) <[^>]*> ac620000 	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 1000ffff 	b	\1 <.*>
+([0-9a-f]+) <[^>]*> 00000000 	nop
+([0-9a-f]+) <[^>]*> ac620000 	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 1000ffff 	b	\1 <.*>
+([0-9a-f]+) <[^>]*> 00000000 	nop
+([0-9a-f]+) <[^>]*> ac620000 	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 1000ffff 	b	\1 <.*>
+([0-9a-f]+) <[^>]*> 00000000 	nop
+([0-9a-f]+) <[^>]*> ac620000 	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 1000ffff 	b	\1 <.*>
+([0-9a-f]+) <[^>]*> 00000000 	nop
+	\.\.\.
+	\.\.\.
Index: binutils-fsf-trunk-quilt/gas/testsuite/gas/mips/branch-self.s
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ binutils-fsf-trunk-quilt/gas/testsuite/gas/mips/branch-self.s	2010-12-08 00:26:16.000000000 +0000
@@ -0,0 +1,36 @@
+# Source file used to test branches to self.
+	.text
+foo:
+	sw	$2, ($3)
+	b	.
+
+	sw	$2, ($3)
+0:
+	b	0b
+
+	sw	$2, ($3)
+bar:
+	b	bar
+
+	sw	$2, ($3)
+	.set	frob, .
+	b	frob
+
+	.eqv	fnord, .
+	sw	$2, ($3)
+	b	fnord
+
+	.eqv	foobar, fnord + 4
+	.eqv	foobaz, foobar - 16
+	sw	$2, ($3)
+	b	foobaz + 12
+
+# Force at least 8 (non-delay-slot) zero bytes, to make 'objdump' print ...
+	.align	2
+	.space	8
+
+# Move the location counter away from the end of code to avoid the final
+# values of symbols equated to expressions involving the counter interfering
+# with disassembly.
+	.align	4
+	.space	16
Index: binutils-fsf-trunk-quilt/gas/testsuite/gas/mips/mips.exp
===================================================================
--- binutils-fsf-trunk-quilt.orig/gas/testsuite/gas/mips/mips.exp	2010-12-08 00:26:16.000000000 +0000
+++ binutils-fsf-trunk-quilt/gas/testsuite/gas/mips/mips.exp	2010-12-08 00:26:16.000000000 +0000
@@ -390,7 +390,7 @@ mips_arch_create mips64r2 64	mips64	{ mi
 			{ -mmips:isa64r2 } \
 			{ mipsisa64r2-*-* mipsisa64r2el-*-* }
 mips_arch_create mips16	32	{}	{} \
-			{ -march=mips1 -mips16 } { -mmips:16 }
+			{ -march=mips1 -mips16 } {}
 mips_arch_create r3000 	32	mips1	{} \
 			{ -march=r3000 -mtune=r3000 } { -mmips:3000 }
 mips_arch_create r3900 	32	mips1	{ gpr_ilocks } \
@@ -466,6 +466,7 @@ if { [istarget mips*-*-vxworks*] } {
     run_dump_test_arches "branch-misc-2pic-64" [mips_arch_list_matching mips3]
     run_dump_test "branch-misc-3"
     run_dump_test "branch-swap"
+    run_dump_test_arches "branch-self"	[mips_arch_list_all]
     run_dump_test "div"
 
     if { !$addr32 } {
Index: binutils-fsf-trunk-quilt/gas/testsuite/gas/mips/mips16@branch-self.d
===================================================================
--- /dev/null	1970-01-01 00:00:00.000000000 +0000
+++ binutils-fsf-trunk-quilt/gas/testsuite/gas/mips/mips16@branch-self.d	2010-12-08 00:26:16.000000000 +0000
@@ -0,0 +1,24 @@
+#objdump: -dr --prefix-addresses --show-raw-insn
+#name: MIPS branches to self
+#as: -32
+#source: branch-self.s
+
+# Test various ways to request a branch to self (MIPS16).
+
+.*: +file format .*mips.*
+
+Disassembly of section \.text:
+([0-9a-f]+) <[^>]*> db40      	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 17ff      	b	\1 <.*>
+([0-9a-f]+) <[^>]*> db40      	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 17ff      	b	\1 <.*>
+([0-9a-f]+) <[^>]*> db40      	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 17ff      	b	\1 <.*>
+([0-9a-f]+) <[^>]*> db40      	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 17ff      	b	\1 <.*>
+([0-9a-f]+) <[^>]*> db40      	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 17ff      	b	\1 <.*>
+([0-9a-f]+) <[^>]*> db40      	sw	v0,0\(v1\)
+([0-9a-f]+) <[^>]*> 17ff      	b	\1 <.*>
+	\.\.\.
+	\.\.\.
Index: binutils-fsf-trunk-quilt/gas/doc/internals.texi
===================================================================
--- binutils-fsf-trunk-quilt.orig/gas/doc/internals.texi	2010-12-08 00:26:16.000000000 +0000
+++ binutils-fsf-trunk-quilt/gas/doc/internals.texi	2010-12-08 10:03:00.000000000 +0000
@@ -1562,6 +1562,12 @@ If defined, GAS will check this macro be
 the DWARF call frame debug information that is emitted.  Targets which
 implement link time relaxation may need to define this macro and set it to zero
 if it is possible to change the size of a function's prologue.
+
+@item TC_PROPAGATE_SYMBOL_ATTRIBUTES
+@cindex TC_PROPAGATE_SYMBOL_ATTRIBUTES
+You should define this macro to copy object format specific information from
+one symbol to another.  GAS will call it when one symbol is set to a sum of
+another and a constant offset.
 @end table
 
 @node Object format backend