ping
Contrary to all documentation, SLOW_BYTE_ACCESS simply means accessing
bitfields by their declared type, which results in better codegeneration
on practically any target. So set it correctly to 1 on Arm.
As a result we generate much better code for bitfields:
typedef struct
{
int x : 2, y : 8, z : 2;
} X;
int bitfield (X *p)
{
return p->x + p->y + p->z;
}
Before:
ldrb r3, [r0] @ zero_extendqisi2
ldrh r2, [r0]
ldrb r0, [r0, #1] @ zero_extendqisi2
sbfx r3, r3, #0, #2
sbfx r2, r2, #2, #8
sbfx r0, r0, #2, #2
sxtab r3, r2, r3
sxtab r0, r3, r0
bx lr
After:
ldr r0, [r0]
sbfx r3, r0, #0, #2
sbfx r2, r0, #2, #8
sbfx r0, r0, #10, #2
sxtab r3, r2, r3
add r0, r0, r3
bx lr
Bootstrap OK, OK for commit?
ChangeLog:
2019-09-11 Wilco Dijkstra <wdijk...@arm.com>
* config/arm/arm.h (SLOW_BYTE_ACCESS): Set to 1.
--
diff --git a/gcc/config/arm/arm.h b/gcc/config/arm/arm.h
index
8b92c830de09a3ad49420fdfacde02d8efc2a89b..11212d988a0f56299c2266bace80170d074be56c
100644
--- a/gcc/config/arm/arm.h
+++ b/gcc/config/arm/arm.h
@@ -1892,8 +1892,9 @@ enum arm_auto_incmodes
((arm_arch4 || (MODE) == QImode) ? ZERO_EXTEND \
: ((BYTES_BIG_ENDIAN && (MODE) == HImode) ? SIGN_EXTEND : UNKNOWN)))
-/* Nonzero if access to memory by bytes is slow and undesirable. */
-#define SLOW_BYTE_ACCESS 0
+/* Contrary to all documentation, this enables wide bitfield accesses,
+ which results in better code when accessing multiple bitfields. */
+#define SLOW_BYTE_ACCESS 1
/* Immediate shift counts are truncated by the output routines (or was it
the assembler?). Shift counts in a register are truncated by ARM. Note
