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RISC-V: Set SLOW_BYTE_ACCESS=1

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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
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Andrew Waterman 2017-11-05 00:30:40 +00:00 • committed by Palmer Dabbelt
parent d327feb3d7
commit ecc82a8d05
2 changed files with 10 additions and 1 deletions
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4
gcc/ChangeLog
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@ -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

7
gcc/config/riscv/riscv.h
View file

@ -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