x86 - add speculation_barrier pattern

This patch adds a speculation barrier for x86, based on my
understanding of the required mitigation for that CPU, which is to use
an lfence instruction.

This patch needs some review by an x86 expert and if adjustments are
needed, I'd appreciate it if they could be picked up by the port
maintainer.  This is supposed to serve as an example of how to deploy
the new __builtin_speculation_safe_value() intrinsic on this
architecture.

	* config/i386/i386.md (unspecv): Add UNSPECV_SPECULATION_BARRIER.
	(speculation_barrier): New insn.

From-SVN: r263196

gcc/ChangeLog

+2018-08-01  Richard Earnshaw  <rearnsha@arm.com>
+
+	* config/i386/i386.md (unspecv): Add UNSPECV_SPECULATION_BARRIER.
+	(speculation_barrier): New insn.
+
 2018-08-01  Richard Biener  <rguenther@suse.de>
 
 	PR bootstrap/86724

gcc/config/i386/i386.md

@@ -301,6 +301,9 @@
 
   ;; For CLDEMOTE support
   UNSPECV_CLDEMOTE
+
+  ;; For Speculation Barrier support
+  UNSPECV_SPECULATION_BARRIER
 ])
 
 ;; Constants to represent rounding modes in the ROUND instruction
@@ -20979,6 +20982,13 @@
   [(set_attr "type" "other")
    (set_attr "memory" "unknown")])
 
+(define_insn "speculation_barrier"
+  [(unspec_volatile [(const_int 0)] UNSPECV_SPECULATION_BARRIER)]
+  ""
+  "lfence"
+  [(set_attr "type" "other")
+   (set_attr "length" "3")])
+
 (include "mmx.md")
 (include "sse.md")
 (include "sync.md")