[Pixman] [PATCH 5/6] Faster write-back for the C variant of r5g6b5 dest iterator

[Siarhei Siamashka]

Unrolling loops improves performance, so just use it here.

Also GCC can't properly optimize this code for RISC processors and
allocate 0x1F001F constant in a register. Because this constant is
too large to be represented as an immediate operand in instructions,
GCC inserts some redundant arithmetics. This problem can be workarounded
by explicitly using a variable for 0x1F001F constant and also initializing
it by a read from another volatile variable. In this case GCC is forced
to allocate a register for it, because it is not seen as a constant anymore.

The speedup relative to the generic store_scanline_r5g6b5() from
"pixman-access.c" (pixman was compiled with gcc 4.7.2):

    MIPS 74K        480MHz  :  33.22 MPix/s ->  43.42 MPix/s
    ARM11           700MHz  :  50.16 MPix/s ->  78.23 MPix/s
    ARM Cortex-A8  1000MHz  : 117.75 MPix/s -> 196.34 MPix/s
    ARM Cortex-A9  1700MHz  : 177.04 MPix/s -> 320.32 MPix/s
    ARM Cortex-A15 1700MHz  : 231.44 MPix/s -> 261.64 MPix/s
    IBM Cell PPU   3200MHz  : 130.25 MPix/s -> 145.61 MPix/s
    Intel Core i7  2800MHz  : 502.21 MPix/s -> 721.73 MPix/s

That's the performance for C code (SIMD and assembly optimizations
are disabled via PIXMAN_DISABLE environment variable).
---
 pixman/pixman-fast-path.c |   38 +++++++++++++++++++++++++++++++++++---
 1 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/pixman/pixman-fast-path.c b/pixman/pixman-fast-path.c
index 4215606..4aa6d73 100644
--- a/pixman/pixman-fast-path.c
+++ b/pixman/pixman-fast-path.c
@@ -2192,17 +2192,49 @@ fast_dest_fetch_noop (pixman_iter_t *iter, const uint32_t *mask)
     return iter->buffer;
 }
 
+/* Helper function for a workaround, which tries to ensure that 0x1F001F
+ * constant is always allocated in a register on RISC architectures.
+ */
+static force_inline uint32_t
+convert_8888_to_0565_workaround (uint32_t s, uint32_t x1F001F)
+{
+    uint32_t a, b;
+    a = (s >> 3) & x1F001F;
+    b = s & 0xFC00;
+    a |= a >> 5;
+    a |= b >> 5;
+    return a;
+}
+
 static void
 fast_write_back_r5g6b5 (pixman_iter_t *iter)
 {
     int32_t w = iter->width;
     uint16_t *dst = (uint16_t *)(iter->bits - iter->stride);
     const uint32_t *src = iter->buffer;
+    /* Workaround to ensure that x1F001F variable is allocated in a register */
+    static volatile uint32_t volatile_x1F001F = 0x1F001F;
+    uint32_t x1F001F = volatile_x1F001F;
 
-    while (w > 0)
+    while ((w -= 4) >= 0)
     {
-	*dst++ = convert_8888_to_0565 (*src++);
-	w--;
+	uint32_t s1 = *src++;
+	uint32_t s2 = *src++;
+	uint32_t s3 = *src++;
+	uint32_t s4 = *src++;
+	*dst++ = convert_8888_to_0565_workaround (s1, x1F001F);
+	*dst++ = convert_8888_to_0565_workaround (s2, x1F001F);
+	*dst++ = convert_8888_to_0565_workaround (s3, x1F001F);
+	*dst++ = convert_8888_to_0565_workaround (s4, x1F001F);
+    }
+    if (w & 2)
+    {
+	*dst++ = convert_8888_to_0565_workaround (*src++, x1F001F);
+	*dst++ = convert_8888_to_0565_workaround (*src++, x1F001F);
+    }
+    if (w & 1)
+    {
+	*dst = convert_8888_to_0565_workaround (*src, x1F001F);
     }
 }
 
-- 
1.7.8.6