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[PATCH] RISC-V: Set SLOW_BYTE_ACCESS=1
- From: Palmer Dabbelt <palmer at dabbelt dot com>
- To: gcc-patches at gcc dot gnu dot org, Michael Clark <michaeljclark at mac dot com>, pinskia at gmail dot com
- Cc: patches at groups dot riscv dot org, Andrew Waterman <andrew at sifive dot com>
- Date: Fri, 3 Nov 2017 08:34:38 -0700
- Subject: [PATCH] RISC-V: Set SLOW_BYTE_ACCESS=1
- Authentication-results: sourceware.org; auth=none
From: Andrew Waterman <andrew@sifive.com>
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
2017-10-03 Andrew Waterman <andrew@sifive.com>
* config/riscv/riscv.h (SLOW_BYTE_ACCESS): Change to 1.
---
gcc/config/riscv/riscv.h | 7 ++++++-
1 file changed, 6 insertions(+), 1 deletion(-)
diff --git a/gcc/config/riscv/riscv.h b/gcc/config/riscv/riscv.h
index e53555efe82f..a802a3f8cbbb 100644
--- 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
--
2.13.6