[DTrace-devel] [PATCH 5/8] Fix stddev() carryover computation

[Kris Van Hees]

On Tue, Jun 04, 2024 at 02:00:05PM -0400, eugene.loh--- via DTrace-devel wrote:
> From: Eugene Loh <eugene.loh at oracle.com>
> 
> The stddev() aggregation function squares 64-bit data values.  A
> value is split into 32-bit high and low values.  Then, (high + low)
> is squared to produce high*high, 2*high*low, and low*low.  Each is
> managed in its own 64-bit register, with the final result residing
> in two 64-bit registers.
> 
> When the 2*high*low portion is combined with the low*low portion,
> care is exercised in case the combination has a carryover portion to
> the higher bits.  This check was broken in the case where low==0.
> That is, data values whose lowest 32 bits were 0 resulted in
> outrageously bad stddev() results.
> 
> Fix the check and add a test for such cases.
> 
> Signed-off-by: Eugene Loh <eugene.loh at oracle.com>

Reviewed-by: Kris Van Hees <kris.van.hees at oracle.com>

> ---
>  libdtrace/dt_cg.c                |  2 +-
>  test/unittest/aggs/tst.stddev2.d | 45 ++++++++++++++++++++++++++++++++
>  test/unittest/aggs/tst.stddev2.r | 13 +++++++++
>  3 files changed, 59 insertions(+), 1 deletion(-)
>  create mode 100644 test/unittest/aggs/tst.stddev2.d
>  create mode 100644 test/unittest/aggs/tst.stddev2.r
> 
> diff --git a/libdtrace/dt_cg.c b/libdtrace/dt_cg.c
> index a1c24e37..fc2cebf0 100644
> --- a/libdtrace/dt_cg.c
> +++ b/libdtrace/dt_cg.c
> @@ -8290,7 +8290,7 @@ dt_cg_agg_stddev(dt_pcb_t *pcb, dt_ident_t *aid, dt_node_t *dnp,
>  	/* Add low value part from mid to lowreg */
>  	emit(dlp,  BPF_ALU64_REG(BPF_ADD, lowreg, lmdreg));
>  	/* Handle the overflow/carry case */
> -	emit(dlp,  BPF_BRANCH_REG(BPF_JLT, lmdreg, lowreg, Lncy));
> +	emit(dlp,  BPF_BRANCH_REG(BPF_JLE, lmdreg, lowreg, Lncy));
>  	emit(dlp,  BPF_ALU64_IMM(BPF_ADD, hi_reg, 1)) /* account for carry */;
>  
>  	/* Sum high value; no overflow expected nor accounted for */
> diff --git a/test/unittest/aggs/tst.stddev2.d b/test/unittest/aggs/tst.stddev2.d
> new file mode 100644
> index 00000000..994bc3e2
> --- /dev/null
> +++ b/test/unittest/aggs/tst.stddev2.d
> @@ -0,0 +1,45 @@
> +/*
> + * Oracle Linux DTrace.
> + * Copyright (c) 2024, Oracle and/or its affiliates. All rights reserved.
> + * Licensed under the Universal Permissive License v 1.0 as shown at
> + * http://oss.oracle.com/licenses/upl.
> + */
> +
> +/*
> + * ASSERTION: Positive stddev() test
> + *
> + * SECTION: Aggregations/Aggregations
> + *
> + * NOTES: This is a simple verifiable positive test of the stddev() function.
> + */
> +
> +#pragma D option quiet
> +
> +BEGIN
> +{
> +	@a = stddev(          0); @a = stddev(          0);
> +	@b = stddev(       0x10); @b = stddev(       0x20);
> +	@c = stddev(      0x100); @c = stddev(      0x200);
> +	@d = stddev(     0x1000); @d = stddev(     0x2000);
> +	@e = stddev(    0x10000); @e = stddev(    0x20000);
> +	@f = stddev(   0x100000); @f = stddev(   0x200000);
> +	@g = stddev(  0x1000000); @g = stddev(  0x2000000);
> +	@h = stddev( 0x10000000); @h = stddev( 0x20000000);
> +	@i = stddev( 0x20000000); @i = stddev( 0x40000000);
> +	@j = stddev( 0x40000000); @j = stddev( 0x80000000);
> +	@k = stddev( 0x80000000); @k = stddev(0x100000000);
> +	@l = stddev(0x100000000); @l = stddev(0x200000000);
> +	printa("%9 at x\n", @a);
> +	printa("%9 at x\n", @b);
> +	printa("%9 at x\n", @c);
> +	printa("%9 at x\n", @d);
> +	printa("%9 at x\n", @e);
> +	printa("%9 at x\n", @f);
> +	printa("%9 at x\n", @g);
> +	printa("%9 at x\n", @h);
> +	printa("%9 at x\n", @i);
> +	printa("%9 at x\n", @j);
> +	printa("%9 at x\n", @k);
> +	printa("%9 at x\n", @l);
> +	exit(0);
> +}
> diff --git a/test/unittest/aggs/tst.stddev2.r b/test/unittest/aggs/tst.stddev2.r
> new file mode 100644
> index 00000000..16e17736
> --- /dev/null
> +++ b/test/unittest/aggs/tst.stddev2.r
> @@ -0,0 +1,13 @@
> +
> +        0
> +        8
> +       80
> +      800
> +     8000
> +    80000
> +   800000
> +  8000000
> + 10000000
> + 20000000
> + 40000000
> + 80000000
> -- 
> 2.18.4
> 
> 
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