MIPS: csum_partial: Improve instruction parallelism.

Computing sum introduces true data dependency. This patch removes some
true data depdendencies, hence increases instruction level parallelism.

This patch brings up to 50% csum performance gain on Loongson 3a.

One example about how this patch works is in CSUM_BIGCHUNK1:
// ** original **    vs    ** patch applied **
    ADDC(sum, t0)           ADDC(t0, t1)
    ADDC(sum, t1)           ADDC(t2, t3)
    ADDC(sum, t2)           ADDC(sum, t0)
    ADDC(sum, t3)           ADDC(sum, t2)

In the original implementation, each ADDC(sum, ...) depends on the sum
value updated by previous ADDC(as source operand).

With this patch applied, the first two ADDC operations are independent,
hence can be executed simultaneously if possible.

Another example is in the "copy and sum calculating chunk":
// ** original **    vs    ** patch applied **
    STORE(t0, UNIT(0) ...   STORE(t0, UNIT(0) ...
    ADDC(sum, t0)           ADDC(t0, t1)
    STORE(t1, UNIT(1) ...   STORE(t1, UNIT(1) ...
    ADDC(sum, t1)           ADDC(sum, t0)
    STORE(t2, UNIT(2) ...   STORE(t2, UNIT(2) ...
    ADDC(sum, t2)           ADDC(t2, t3)
    STORE(t3, UNIT(3) ...   STORE(t3, UNIT(3) ...
    ADDC(sum, t3)           ADDC(sum, t2)

With this patch applied, ADDC and the **next next** ADDC are independent.

Signed-off-by: chenj <chenj@lemote.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/9608/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

diff --git a/arch/mips/lib/csum_partial.S b/arch/mips/lib/csum_partial.S
index 4c721e247ac9a001f4cc4c2335da851143ae1e8a..ed88647b57e26710e9a720d9b4cb9aede0def430 100644 (file)
--- a/arch/mips/lib/csum_partial.S
+++ b/arch/mips/lib/csum_partial.S
@@ -76,10 +76,10 @@
        LOAD    _t1, (offset + UNIT(1))(src);                   \
        LOAD    _t2, (offset + UNIT(2))(src);                   \
        LOAD    _t3, (offset + UNIT(3))(src);                   \
+       ADDC(_t0, _t1);                                         \
+       ADDC(_t2, _t3);                                         \
        ADDC(sum, _t0);                                         \
-       ADDC(sum, _t1);                                         \
-       ADDC(sum, _t2);                                         \
-       ADDC(sum, _t3)
+       ADDC(sum, _t2)
 
 #ifdef USE_DOUBLE
 #define CSUM_BIGCHUNK(src, offset, sum, _t0, _t1, _t2, _t3)    \
@@ -504,21 +504,21 @@ LEAF(csum_partial)
        SUB     len, len, 8*NBYTES
        ADD     src, src, 8*NBYTES
        STORE(t0, UNIT(0)(dst), .Ls_exc\@)
-       ADDC(sum, t0)
+       ADDC(t0, t1)
        STORE(t1, UNIT(1)(dst), .Ls_exc\@)
-       ADDC(sum, t1)
+       ADDC(sum, t0)
        STORE(t2, UNIT(2)(dst), .Ls_exc\@)
-       ADDC(sum, t2)
+       ADDC(t2, t3)
        STORE(t3, UNIT(3)(dst), .Ls_exc\@)
-       ADDC(sum, t3)
+       ADDC(sum, t2)
        STORE(t4, UNIT(4)(dst), .Ls_exc\@)
-       ADDC(sum, t4)
+       ADDC(t4, t5)
        STORE(t5, UNIT(5)(dst), .Ls_exc\@)
-       ADDC(sum, t5)
+       ADDC(sum, t4)
        STORE(t6, UNIT(6)(dst), .Ls_exc\@)
-       ADDC(sum, t6)
+       ADDC(t6, t7)
        STORE(t7, UNIT(7)(dst), .Ls_exc\@)
-       ADDC(sum, t7)
+       ADDC(sum, t6)
        .set    reorder                         /* DADDI_WAR */
        ADD     dst, dst, 8*NBYTES
        bgez    len, 1b
@@ -544,13 +544,13 @@ LEAF(csum_partial)
        SUB     len, len, 4*NBYTES
        ADD     src, src, 4*NBYTES
        STORE(t0, UNIT(0)(dst), .Ls_exc\@)
-       ADDC(sum, t0)
+       ADDC(t0, t1)
        STORE(t1, UNIT(1)(dst), .Ls_exc\@)
-       ADDC(sum, t1)
+       ADDC(sum, t0)
        STORE(t2, UNIT(2)(dst), .Ls_exc\@)
-       ADDC(sum, t2)
+       ADDC(t2, t3)
        STORE(t3, UNIT(3)(dst), .Ls_exc\@)
-       ADDC(sum, t3)
+       ADDC(sum, t2)
        .set    reorder                         /* DADDI_WAR */
        ADD     dst, dst, 4*NBYTES
        beqz    len, .Ldone\@
@@ -649,13 +649,13 @@ LEAF(csum_partial)
        nop                             # improves slotting
 #endif
        STORE(t0, UNIT(0)(dst), .Ls_exc\@)
-       ADDC(sum, t0)
+       ADDC(t0, t1)
        STORE(t1, UNIT(1)(dst), .Ls_exc\@)
-       ADDC(sum, t1)
+       ADDC(sum, t0)
        STORE(t2, UNIT(2)(dst), .Ls_exc\@)
-       ADDC(sum, t2)
+       ADDC(t2, t3)
        STORE(t3, UNIT(3)(dst), .Ls_exc\@)
-       ADDC(sum, t3)
+       ADDC(sum, t2)
        .set    reorder                         /* DADDI_WAR */
        ADD     dst, dst, 4*NBYTES
        bne     len, rem, 1b