[PATCH] Use more conservative fences on RISC-V

Palmer Dabbelt palmer@dabbelt.com
Fri Mar 17 22:56:00 GMT 2017


The RISC-V memory model is still in the process of being formally
specified, so for now we're going to be safe and add the I/O bits to
userspace fences because there's no way to know if userspace is touching
memory-mapped I/O regions at compile time.

This will have no impact on existing microarchitecutres because they
treat all fences conservatively.

gcc/ChangeLog:

2017-03-17  Palmer Dabbelt  <palmer@dabbelt.com>

        * config/riscv/riscv.c (riscv_print_operand): Use "fence
        iorw,ow".
        * config/riscv/sync.mc (mem_thread_fence_1): Use "fence
        iorw,iorw".
---
 gcc/ChangeLog            | 7 +++++++
 gcc/config/riscv/riscv.c | 2 +-
 gcc/config/riscv/sync.md | 2 +-
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index 3e108dd..de32689 100644
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,10 @@
+2017-03-17  Palmer Dabbelt  <palmer@dabbelt.com>
+
+	* config/riscv/riscv.c (riscv_print_operand): Use "fence
+	iorw,ow".
+	* config/riscv/sync.mc (mem_thread_fence_1): Use "fence
+	iorw,iorw".
+
 2017-03-17  Palmer Dabbelt  <palmer@dabbelt.com
 
 	* doc/install.texi (Specific) <riscv32-*-elf>: Add riscv32-*-elf,
diff --git a/gcc/config/riscv/riscv.c b/gcc/config/riscv/riscv.c
index 25cc803..fa93c3c 100644
--- a/gcc/config/riscv/riscv.c
+++ b/gcc/config/riscv/riscv.c
@@ -2794,7 +2794,7 @@ riscv_print_operand (FILE *file, rtx op, int letter)
 
     case 'F':
       if (riscv_memmodel_needs_release_fence ((enum memmodel) INTVAL (op)))
-	fputs ("fence rw,w; ", file);
+	fputs ("fence iorw,ow; ", file);
       break;
 
     default:
diff --git a/gcc/config/riscv/sync.md b/gcc/config/riscv/sync.md
index 09970b9..cde19e3 100644
--- a/gcc/config/riscv/sync.md
+++ b/gcc/config/riscv/sync.md
@@ -53,7 +53,7 @@
 	(unspec:BLK [(match_dup 0)] UNSPEC_MEMORY_BARRIER))
    (match_operand:SI 1 "const_int_operand" "")] ;; model
   ""
-  "fence\trw,rw")
+  "fence\tiorw,iorw")
 
 ;; Atomic memory operations.
 
-- 
2.10.2



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