[ARM] Allow Thumb division as an extension for ARMv7

=== Context ===

This patch is part of a patch series to add support for ARMv8-R
architecture. Its purpose is to allow ARMv7 to be selected in automatic
architecture selection in presence of Thumb division instructions.

=== Motivation ===

any-idiv.d and automatic-sdiv.d testcases in GAS testsuite expect
autodetection code to select ARMv7 in presence of Thumb integer
division. However, the definition of ARM_AEXT_V7 and thus ARM_ARCH_V7 do
not contain these instructions and the idiv extension is only available
for ARMv7-A and ARMv7-R. Therefore, under the stricter automatic
detection code proposed in the subsequent patch of the series ARMv7 is
refused if a Thumb division instruction is present.

=== Patch description ===

This patch adds a new "idiv" extension after the existing one that is
available to all ARMv7 targets. This new entry is ignored by all current
code parsing arm_extensions such that it would be unavailable on the
command-line and remain a purely internal hack, easily removed in favor
of a better solution later. This is considered though by the subsequent
patch reworking automatic detection of build attributes such that ARMv7
is allowed to match in present of Thumb division instructions. For good
measure, comments are added in all instances of code browsing
arm_extensions to mention the expected behavior in case of duplicate
entries as well as a new testcase.

2017-06-20  Thomas Preud'homme  <thomas.preudhomme@arm.com>

gas/
	* config/tc-arm.c (arm_extensions): New duplicate idiv entry to enable
	Thumb division for ARMv7 architecture.
	(arm_parse_extension): Document expected behavior for duplicate
	entries.
	(s_arm_arch_extension): Likewise.
	* testsuite/gas/arm/forbid-armv7-idiv-ext.d: New test.
	* testsuite/gas/arm/forbid-armv7-idiv-ext.l: New expected output for
	above test.

4 files changed, 33 insertions, 0 deletions

diff --git a/gas/ChangeLog b/gas/ChangeLog
index 82b25ac..8c62992 100644
--- a/gas/ChangeLog
+++ b/gas/ChangeLog
@@ -1,3 +1,14 @@
+2017-06-20  Thomas Preud'homme  <thomas.preudhomme@arm.com>
+
+	* config/tc-arm.c (arm_extensions): New duplicate idiv entry to enable
+	Thumb division for ARMv7 architecture.
+	(arm_parse_extension): Document expected behavior for duplicate
+	entries.
+	(s_arm_arch_extension): Likewise.
+	* testsuite/gas/arm/forbid-armv7-idiv-ext.d: New test.
+	* testsuite/gas/arm/forbid-armv7-idiv-ext.l: New expected output for
+	above test.
+
 2017-06-21  Thomas Preud'homme  <thomas.preudhomme@arm.com>
 
 	* config/tc-arm.c (aeabi_set_public_attributes): Populate flags from
diff --git a/gas/config/tc-arm.c b/gas/config/tc-arm.c
index 86acb86..a62c5f2 100644
--- a/gas/config/tc-arm.c
+++ b/gas/config/tc-arm.c
@@ -25963,6 +25963,13 @@ static const struct arm_option_extension_value_table arm_extensions[] =
 			ARM_FEATURE_CORE_LOW (ARM_EXT_ADIV | ARM_EXT_DIV),
 			ARM_FEATURE_CORE_LOW (ARM_EXT_V7A),
 			ARM_FEATURE_CORE_LOW (ARM_EXT_V7R)),
+  /* Duplicate entry for the purpose of allowing ARMv7 to match in presence of
+     Thumb divide instruction.  Due to this having the same name as the
+     previous entry, this will be ignored when doing command-line parsing and
+     only considered by build attribute selection code.  */
+  ARM_EXT_OPT ("idiv",	ARM_FEATURE_CORE_LOW (ARM_EXT_DIV),
+			ARM_FEATURE_CORE_LOW (ARM_EXT_DIV),
+			ARM_FEATURE_CORE_LOW (ARM_EXT_V7)),
   ARM_EXT_OPT ("iwmmxt",ARM_FEATURE_COPROC (ARM_CEXT_IWMMXT),
 			ARM_FEATURE_COPROC (ARM_CEXT_IWMMXT), ARM_ARCH_NONE),
   ARM_EXT_OPT ("iwmmxt2", ARM_FEATURE_COPROC (ARM_CEXT_IWMMXT2),
@@ -26195,6 +26202,10 @@ arm_parse_extension (const char *str, const arm_feature_set *opt_set,
 	    else
 	      ARM_CLEAR_FEATURE (**ext_set_p, **ext_set_p, opt->clear_value);
 
+	    /* Allowing Thumb division instructions for ARMv7 in autodetection
+	       rely on this break so that duplicate extensions (extensions
+	       with the same name as a previous extension in the list) are not
+	       considered for command-line parsing.  */
 	    break;
 	  }
 
@@ -27028,6 +27039,10 @@ s_arm_arch_extension (int ignored ATTRIBUTE_UNUSED)
 	ARM_MERGE_FEATURE_SETS (cpu_variant, selected_cpu, *mfpu_opt);
 	*input_line_pointer = saved_char;
 	demand_empty_rest_of_line ();
+	/* Allowing Thumb division instructions for ARMv7 in autodetection rely
+	   on this return so that duplicate extensions (extensions with the
+	   same name as a previous extension in the list) are not considered
+	   for command-line parsing.  */
 	return;
       }
 
diff --git a/gas/testsuite/gas/arm/forbid-armv7-idiv-ext.d b/gas/testsuite/gas/arm/forbid-armv7-idiv-ext.d
new file mode 100644
index 0000000..85c7dc3
--- /dev/null
+++ b/gas/testsuite/gas/arm/forbid-armv7-idiv-ext.d
@@ -0,0 +1,4 @@
+# name: Forbidden idiv for ARMv7
+# source: blank.s
+# as: -march=armv7+idiv
+#error-output: forbid-armv7-idiv-ext.l
diff --git a/gas/testsuite/gas/arm/forbid-armv7-idiv-ext.l b/gas/testsuite/gas/arm/forbid-armv7-idiv-ext.l
new file mode 100644
index 0000000..76208d2
--- /dev/null
+++ b/gas/testsuite/gas/arm/forbid-armv7-idiv-ext.l
@@ -0,0 +1,3 @@
+Assembler messages:
+[^:]*: extension does not apply to the base architecture
+[^:]*: unrecognized option -march=armv7\+idiv