From: Paul Koning <paulkoning@comcast.net>
To: GCC Development <gcc@gcc.gnu.org>
Subject: Inefficient code
Date: Thu, 05 Jul 2018 12:46:00 -0000 [thread overview]
Message-ID: <FB612838-028B-491B-82DB-6A334D8E8472@comcast.net> (raw)
I have a struct that looks like this:
struct Xrb
{
uint16_t xrlen; /* Length of I/O buffer in bytes */
uint16_t xrbc; /* Byte count for transfer */
void * xrloc; /* Pointer to I/O buffer */
uint8_t xrci; /* Channel number times 2 for transfer */
uint32_t xrblk:24; /* Random access block number */
uint16_t xrtime; /* Wait time for terminal input */
uint16_t xrmod; /* Modifiers */
};
When I write to xrblk (that 24 bit field) on my 16 bit target, I get unexpectly inefficient output:
XRB->xrblk = 5;
movb #5,10(r0)
clrb 11(r0)
clrb 7(r0)
rather than the expected word write to the word-aligned lower half of that field.
Looking at the dumps, I see it coming into the RTL expand phase as a single write, which expand then turns into the three insns corresponding to the above. But (of course) there is a word (HImode) move also, which has the same cost as the byte one.
Is there something I have to do in my target definition to get this to come out right? This is a strict_alignment target, but alignment is satisfied in this example. Also, SLOW_BYTE_ACCESS is 1.
paul
next reply other threads:[~2018-07-05 12:46 UTC|newest]
Thread overview: 11+ messages / expand[flat|nested] mbox.gz Atom feed top
2018-07-05 12:46 Paul Koning [this message]
2018-07-05 16:01 ` Segher Boessenkool
2018-07-05 16:29 ` Paul Koning
2018-07-05 20:44 ` Eric Botcazou
2018-07-05 20:53 ` Paul Koning
2018-07-05 22:47 ` Eric Botcazou
2018-07-06 1:01 ` Paul Koning
2018-07-06 1:04 ` Paul Koning
2018-07-06 6:54 ` Eric Botcazou
2018-07-06 10:18 Bernd Edlinger
2018-07-06 12:55 ` Paul Koning
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