For reference. We know that the NEON intrinsics in GCC have issues.
I came across this page:
http://hilbert-space.de/?p=22
which has a colour to greyscale conversion done using intrinsics.
gcc-linaro-4.5-2011.03-0 does poorly through saving intermediate
values on the stack. The core of the loop is:
.L3:
mov ip, r4
vld3.8 {d16-d18}, [r6]
vstmia r4, {d16-d18}
ldmia ip!, {r0, r1, r2, r3}
mov sl, r9
adds r7, r7, #1
adds r6, r6, #24
stmia sl!, {r0, r1, r2, r3}
fldd d16, [sp, #24]
fldd d18, [sp, #32]
ldmia ip, {r0, r1}
vmull.u8 q8, d16, d19
stmia sl, {r0, r1}
vmlal.u8 q8, d18, d20
fldd d18, [sp, #40]
vmlal.u8 q8, d18, d21
vshrn.i16 d16, q8, #8
vst1.8 {d16}, [r5]
adds r5, r5, #8
cmp r8, r7
bgt .L3
llvm-2.9~svn128540 does much better:
vld3.8 {d20, d21, d22}, [r1]!
add r3, r3, #1
cmp r3, r2
vmull.u8 q12, d21, d16
vmlal.u8 q12, d20, d17
vmlal.u8 q12, d22, d18
vshrn.i16 d19, q12, #8
vst1.8 {d19}, [r0]!
blt .LBB0_1
and may actually be better than the had-written assembler on Nils's
page due to scheduling the loop comparison earlier.
Richard S, were you looking into this?
-- Michael
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