One of the vectorisation discussions from last year was about the poor
code GCC generates for vld{2,3,4}_*() and vst{2,3,4}_*(). It forces the
result of the loads onto the stack, then loads the individual pieces from
there. It does the same thing in reverse for stores.
I think there are two major problems here:
1. The result of the vld*() is a record type such as:
typedef struct int16x4x3_t
{
int16x4_t val[3];
} int16x4x3_t;
Ideally, we'd like one of these structures to be stored in a pseudo
register. However, the ARM port currently limits in-register
record types to 64 bits, so something this big is always given
BLKmode and stored on the stack.
A simple "fix" for this is to increase MAX_FIXED_MODE_SIZE.
That would do the right thing for the structures in arm_neon.h,
but wouldn't be safe in general.
2. The vld*() returns values as a single integer (such as EI mode),
while uses of the value will typically be in a vector mode such
as V4SI. CANNOT_CHANGE_MODE_CLASS doesn't allow direct
"mode-punning" between the two in VFP_REGS, so this again
forces the punning to be done on the stack.
The code in question is:
/* FPA registers can't do subreg as all values are reformatted to internal
precision. VFP registers may only be accessed in the mode they
were set. */
#define CANNOT_CHANGE_MODE_CLASS(FROM, TO, CLASS) \
(GET_MODE_SIZE (FROM) != GET_MODE_SIZE (TO) \
? reg_classes_intersect_p (FPA_REGS, (CLASS)) \
|| reg_classes_intersect_p (VFP_REGS, (CLASS)) \
However, the VFP restriction appears to be specific to VFPv1 --
thanks to Peter for the archaeology -- and isn't a problem for v6+.
In that case, removing this restriction is an important optimisation.
I tried the patch below on the following simple testcase:
#include "arm_neon.h"
void
foo (uint16_t *a)
{
uint16x4x3_t x, y;
x = vld3_u16 (a);
y = vld3_u16 (a + 12);
x.val[0] = vadd_u16 (x.val[0], y.val[0]);
x.val[1] = vadd_u16 (x.val[1], y.val[1]);
x.val[2] = vadd_u16 (x.val[2], y.val[2]);
vst3_u16 (a, x);
}
(not necessarily sensible!). Before the patch, -O2 produced:
sub sp, sp, #48
add r3, r0, #24
vld3.16 {d16-d18}, [r3]
vld3.16 {d20-d22}, [r0]
add r3, sp, #24
vstmia sp, {d20-d22}
vstmia r3, {d16-d18}
fldd d19, [sp, #8]
fldd d16, [sp, #0]
fldd d17, [sp, #24]
fldd d20, [sp, #32]
vadd.i16 d18, d16, d17
vadd.i16 d17, d19, d20
fldd d19, [sp, #16]
fldd d20, [sp, #40]
vadd.i16 d16, d19, d20
fstd d18, [sp, #0]
fstd d17, [sp, #8]
fstd d16, [sp, #16]
vldmia sp, {d16-d18}
vst3.16 {d16-d18}, [r0]
add sp, sp, #48
bx lr
After the patch we get:
vld3.16 {d24-d26}, [r0]
add r3, r0, #24
vld3.16 {d20-d22}, [r3]
vmov q8, q12 @ ti
vadd.i16 d17, d17, d21
vadd.i16 d16, d24, d20
vadd.i16 d18, d26, d22
vst3.16 {d16-d18}, [r0]
bx lr
The VMOV is a bit disappointing, and needs further investigation.
The first hunk fixes (2), and I think is correct. The second hunk
hacks (1), and isn't suitable in itself. I'll next try to make
arm_neon.h use built-in record types that are explicitly EImode,
which should remove the need to change MAX_FIXED_MODE_SIZE.
Richard
Index: gcc/gcc/config/arm/arm.h
===================================================================
--- gcc.orig/gcc/config/arm/arm.h
+++ gcc/gcc/config/arm/arm.h
@@ -1171,10 +1171,12 @@ enum reg_class
/* FPA registers can't do subreg as all values are reformatted to internal
precision. VFP registers may only be accessed in the mode they
were set. */
-#define CANNOT_CHANGE_MODE_CLASS(FROM, TO, CLASS) \
- (GET_MODE_SIZE (FROM) != GET_MODE_SIZE (TO) \
- ? reg_classes_intersect_p (FPA_REGS, (CLASS)) \
- || reg_classes_intersect_p (VFP_REGS, (CLASS)) \
2+#define CANNOT_CHANGE_MODE_CLASS(FROM, TO, CLASS) \
+ (GET_MODE_SIZE (FROM) != GET_MODE_SIZE (TO) \
+ ? (reg_classes_intersect_p (FPA_REGS, (CLASS)) \
+ || (TARGET_VFP \
+ && reg_classes_intersect_p (VFP_REGS, (CLASS)) \
+ && arm_fpu_desc->rev == 1)) \
: 0)
/* The class value for index registers, and the one for base regs. */
@@ -2458,4 +2460,6 @@ enum arm_builtins
instruction. */
#define MAX_LDM_STM_OPS 4
+#define MAX_FIXED_MODE_SIZE GET_MODE_BITSIZE (XImode)
+
#endif /* ! GCC_ARM_H */
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