RISC-V: Set SLOW_BYTE_ACCESS=1

When implementing the RISC-V port, I took the name of this macro at
face value.  It appears we were mistaken in what this means, here's a
quote from the SPARC port that better describes what SLOW_BYTE_ACCESS
does

    /* Nonzero if access to memory by bytes is slow and undesirable.
       For RISC chips, it means that access to memory by bytes is no
       better than access by words when possible, so grab a whole word
       and maybe make use of that.  */

I've added the comment to our port as well.

See https://gcc.gnu.org/ml/gcc/2017-08/msg00202.html for more
discussion.  Thanks to Michael Clark and Andrew Pinski for the help!

gcc/ChangeLog

2017-11-04  Andrew Waterman  <andrew@sifive.com>

        * config/riscv/riscv.h (SLOW_BYTE_ACCESS): Change to 1.

From-SVN: r254416

diff --git a/gcc/ChangeLog b/gcc/ChangeLog
index ec8af50fa050804e6285e3de42051db60a0de910..285ac2014df87e429d82beff5301a0888105c094 100644 (file)
--- a/gcc/ChangeLog
+++ b/gcc/ChangeLog
@@ -1,3 +1,7 @@
+2017-11-04  Andrew Waterman  <andrew@sifive.com>
+
+       * config/riscv/riscv.h (SLOW_BYTE_ACCESS): Change to 1.
+
 2017-11-04  Daniel Santos  <daniel.santos@pobox.com>
 
        config/i386/i386.c (choose_basereg): Use optional scratch

diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h
index e53555efe82fbcece81c18adf22bab5c06edef0d..a802a3f8cbbb49791ae9fc1d8df4f28ded41f92e 100644 (file)
--- a/gcc/config/riscv/riscv.h
+++ b/gcc/config/riscv/riscv.h
@@ -615,7 +615,12 @@ typedef struct {
 #define MOVE_MAX UNITS_PER_WORD
 #define MAX_MOVE_MAX 8
 
-#define SLOW_BYTE_ACCESS 0
+/* The SPARC port says:
+   Nonzero if access to memory by bytes is slow and undesirable.
+   For RISC chips, it means that access to memory by bytes is no
+   better than access by words when possible, so grab a whole word
+   and maybe make use of that.  */
+#define SLOW_BYTE_ACCESS 1
 
 #define SHIFT_COUNT_TRUNCATED 1