Add option -mfence-as-lock-add=[no|yes].

With -mfence-as-lock-add=yes lfence, mfence and sfence will be encoded
as lock addl $0x0, (%{r,e}sp).

gas/:

    * config/tc-i386.c (avoid_fence): New.
    (output_insn): Encode as lock addl $0x0, (%{r,e}sp) if avoid_fence
    is true.
    (OPTION_FENCE_AS_LOCK_ADD): New.
    (md_longopts): Add -mfence-as-lock-add.
    (md_parse_option): Handle -mfence-as-lock-add.
    (md_show_usage): Add -mfence-as-lock-add=[no|yes].
    * doc/c-i386.texi (-mfence-as-lock-add): Document.

gas/testsuite/:

    * gas/i386/i386.exp: Run new tests.
    * gas/i386/fence-as-lock-add.s: New.
    * gas/i386/fence-as-lock-add-yes.d: Likewise.
    * gas/i386/fence-as-lock-add-no.d: Likewise.
    * gas/i386/x86-64-fence-as-lock-add-yes.d: Likewise.
    * gas/i386/x86-64-fence-as-lock-add-no.d: Likewise.

6 files changed, 66 insertions, 0 deletions

diff --git a/gas/testsuite/gas/i386/fence-as-lock-add-no.d b/gas/testsuite/gas/i386/fence-as-lock-add-no.d
new file mode 100644
index 0000000..2083066
--- /dev/null
+++ b/gas/testsuite/gas/i386/fence-as-lock-add-no.d
@@ -0,0 +1,14 @@
+#source: fence-as-lock-add.s
+#as: -mfence-as-lock-add=no
+#objdump: -dw
+#name: i386  fence as lock add = no
+
+.*: +file format .*i386.*
+
+Disassembly of section .text:
+
+0+ <main>:
+[   ]*[a-f0-9]+:	0f ae e8[ ]*	lfence 
+[   ]*[a-f0-9]+:	0f ae f0[ ]*	mfence 
+[   ]*[a-f0-9]+:	0f ae f8[ ]*	sfence 
+#pass
diff --git a/gas/testsuite/gas/i386/fence-as-lock-add-yes.d b/gas/testsuite/gas/i386/fence-as-lock-add-yes.d
new file mode 100644
index 0000000..4ee6300
--- /dev/null
+++ b/gas/testsuite/gas/i386/fence-as-lock-add-yes.d
@@ -0,0 +1,14 @@
+#source: fence-as-lock-add.s
+#as: -mfence-as-lock-add=yes
+#objdump: -dw
+#name: i386  fence as lock add = yes
+
+.*: +file format .*i386.*
+
+Disassembly of section .text:
+
+0+ <main>:
+[   ]*[a-f0-9]+:	f0 83 04 24 00[ ]*	lock addl \$0x0,\(%esp\)
+[   ]*[a-f0-9]+:	f0 83 04 24 00[ ]*	lock addl \$0x0,\(%esp\)
+[   ]*[a-f0-9]+:	f0 83 04 24 00[ ]*	lock addl \$0x0,\(%esp\)
+#pass
diff --git a/gas/testsuite/gas/i386/fence-as-lock-add.s b/gas/testsuite/gas/i386/fence-as-lock-add.s
new file mode 100644
index 0000000..5ecb1d9
--- /dev/null
+++ b/gas/testsuite/gas/i386/fence-as-lock-add.s
@@ -0,0 +1,6 @@
+	.text
+.globl main
+main:
+	lfence
+	mfence
+	sfence
diff --git a/gas/testsuite/gas/i386/i386.exp b/gas/testsuite/gas/i386/i386.exp
index e23a4e5..b9144c4 100644
--- a/gas/testsuite/gas/i386/i386.exp
+++ b/gas/testsuite/gas/i386/i386.exp
@@ -303,6 +303,8 @@ if [expr ([istarget "i*86-*-*"] ||  [istarget "x86_64-*-*"]) && [gas_32_check]]
     run_dump_test "avx512dq_vl"
     run_dump_test "omit-lock-yes"
     run_dump_test "omit-lock-no"
+    run_dump_test "fence-as-lock-add-yes"
+    run_dump_test "fence-as-lock-add-no"
     run_dump_test "avx512dq-rcigrd-intel"
     run_dump_test "avx512dq-rcigrd"
     run_dump_test "avx512dq-rcigrne-intel"
@@ -731,6 +733,8 @@ if [expr ([istarget "i*86-*-*"] || [istarget "x86_64-*-*"]) && [gas_64_check]] t
     run_dump_test "x86-64-mwaitx-bdver4"
     run_list_test "x86-64-mwaitx-reg"
     run_dump_test "x86-64-ospke"
+    run_dump_test "x86-64-fence-as-lock-add-yes"
+    run_dump_test "x86-64-fence-as-lock-add-no"
 
     if { ![istarget "*-*-aix*"]
       && ![istarget "*-*-beos*"]
diff --git a/gas/testsuite/gas/i386/x86-64-fence-as-lock-add-no.d b/gas/testsuite/gas/i386/x86-64-fence-as-lock-add-no.d
new file mode 100644
index 0000000..9277448
--- /dev/null
+++ b/gas/testsuite/gas/i386/x86-64-fence-as-lock-add-no.d
@@ -0,0 +1,14 @@
+#source: fence-as-lock-add.s
+#as: -mfence-as-lock-add=no
+#objdump: -dw
+#name: x86-64 fence as lock add = no
+
+.*: +file format .*
+
+Disassembly of section .text:
+
+0+ <main>:
+[   ]*[a-f0-9]+:	0f ae e8[ ]*	lfence 
+[   ]*[a-f0-9]+:	0f ae f0[ ]*	mfence 
+[   ]*[a-f0-9]+:	0f ae f8[ ]*	sfence 
+#pass
diff --git a/gas/testsuite/gas/i386/x86-64-fence-as-lock-add-yes.d b/gas/testsuite/gas/i386/x86-64-fence-as-lock-add-yes.d
new file mode 100644
index 0000000..9451cd8
--- /dev/null
+++ b/gas/testsuite/gas/i386/x86-64-fence-as-lock-add-yes.d
@@ -0,0 +1,14 @@
+#source: fence-as-lock-add.s
+#as: -mfence-as-lock-add=yes
+#objdump: -dw
+#name: x86-64 fence as lock add = yes
+
+.*: +file format .*
+
+Disassembly of section .text:
+
+0+ <main>:
+[   ]*[a-f0-9]+:	f0 83 04 24 00[ ]*	lock addl \$0x0,\(%rsp\)
+[   ]*[a-f0-9]+:	f0 83 04 24 00[ ]*	lock addl \$0x0,\(%rsp\)
+[   ]*[a-f0-9]+:	f0 83 04 24 00[ ]*	lock addl \$0x0,\(%rsp\)
+#pass