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Frob numeric.c loop so that clang will auto-vectorize it too.

Mon, 7 Sep 2020 16:03:04 +0000 (12:03 -0400)

Experimentation shows that clang will auto-vectorize the critical
multiplication loop if the termination condition is written "i2 < limit"
rather than "i2 <= limit". This seems unbelievably stupid, but I've
reproduced it on both clang 9.0.1 (RHEL8) and 11.0.3 (macOS Catalina).
gcc doesn't care, so tweak the code to do it that way.

Discussion: https://postgr.es/m/CAJ3gD9evtA_vBo+WMYMyT-u=keHX7-r8p2w7OSRfXf42LTwCZQ@mail.gmail.com

src/backend/utils/adt/numeric.c patch | blob | blame | history

diff --git a/src/backend/utils/adt/numeric.c b/src/backend/utils/adt/numeric.c

index d2a42b811daca8d707827e0764441fb26c7ea1b0..dfd455fc748816ac588828c7f24cff8030727641 100644 (file)

--- a/src/backend/utils/adt/numeric.c

+++ b/src/backend/utils/adt/numeric.c

@@ -8191,7 +8191,6 @@ mul_var(const NumericVar *var1, const NumericVar *var2, NumericVar *result,

int res_weight;

int maxdigits;

int *dig;

- int *dig_i1_2;

int carry;

int maxdig;

int newdig;

@@ -8327,7 +8326,7 @@ mul_var(const NumericVar *var1, const NumericVar *var2, NumericVar *result,

* Add the appropriate multiple of var2 into the accumulator.

* As above, digits of var2 can be ignored if they don't contribute,

- * so we only include digits for which i1+i2+2 <= res_ndigits - 1.

+ * so we only include digits for which i1+i2+2 < res_ndigits.

* This inner loop is the performance bottleneck for multiplication,

* so we want to keep it simple enough so that it can be

@@ -8336,10 +8335,13 @@ mul_var(const NumericVar *var1, const NumericVar *var2, NumericVar *result,

* Since we aren't propagating carries in this loop, the order does

* not matter.

- i = Min(var2ndigits - 1, res_ndigits - i1 - 3);

- dig_i1_2 = &dig[i1 + 2];

- for (i2 = 0; i2 <= i; i2++)

- dig_i1_2[i2] += var1digit * var2digits[i2];

+ {

+ int i2limit = Min(var2ndigits, res_ndigits - i1 - 2);

+ int *dig_i1_2 = &dig[i1 + 2];

+ for (i2 = 0; i2 < i2limit; i2++)

+ dig_i1_2[i2] += var1digit * var2digits[i2];

+ }

}

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