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Re: Curious buildfarm failures (fwd)

From: Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>
To: Andres Freund <andres(at)2ndquadrant(dot)com>
Cc: Sergey Koposov <koposov(at)ast(dot)cam(dot)ac(dot)uk>, pgsql-hackers(at)postgreSQL(dot)org
Subject: Re: Curious buildfarm failures (fwd)
Date: 2013-01-16 06:28:09
Message-ID: (view raw, whole thread or download thread mbox)
Lists: pgsql-hackers
It's a compiler bug.

icc 11.1 apparently thinks that this loop in doPickSplit:

     * Update nodes[] array to point into the newly formed innerTuple, so that
     * we can adjust their downlinks below.
    SGITITERATE(innerTuple, i, node)
        nodes[i] = node;

is going to be iterated hundreds of times, because it expends a
ridiculous amount of effort on it, starting with a loop prolog that
prefetches about a thousand bytes starting at the nodes pointer :-(
(Why does it think it needs to prefetch an array it's only going to
write into?  Is IA64's cache hardware really that stupid?)
And it makes use of IA64's bizarre scheme for software-unrolling
loops, which I am going to do my darnedest to forget now that I've
learned it; but for the purposes of this bit you only need to know
that the br.wtop.dptk instruction "renames" registers 32 and up,
so that whatever is in r32 when the bottom of the loop is reached
will be in r33 on the next iteration, and r33's contents move to r34,
etc.  In this particular example, this ridiculous complication saves
a grand total of no instructions, but nevermind that.

Before starting the loop, the code has computed

r28 = innerTuple
r29 = nodes
r26 = r29 + 1200 (this is where it will continue the prefetching...)
r33 = 0
r35 = innerTuple + innerTuple->prefixSize + 8 (ie, the initial value of "nt")
r27 = innerTuple + innerTuple->prefixSize + 8 + 6

And the body of the SGITERATE loop looks like

at top of loop, r35 contains "nt" pointer, r33 contains "i"
 (p17)	st8	[r29]=r35,8				//0: {940:3} 4456 0
store nt at *r29, increment r29 by 8 bytes (thus, assign to nodes[i])
 (p17)	add	r32=1,r33				//0: {938:2} 4453 0
compute i+1, will be next value of i due to register rename
 (p17)	ld2	r36=[r28]				//1: {938:2} 4462 0
fetch first 2 bytes of innerTuple
 (p17)	ld2	r34=[r27],r33				//1: {938:2} 4459 0
fetch last 2 bytes of node tuple, on first iteration anyway ...
and then add the value of r33 to r27, which is all wrong
 (p17)	extr.u	r37=r36,3,13				//2: {938:2} 4463 0
extract nNodes from fetched 2 bytes of innerTuple
 (p17)	extr.u	r33=r34,0,13 ;;				//2: {938:2} 4460 0
extract size field of node tuple, or so it hopes
 (p17)	lfetch.nt1	[r26],8				//3: {938:2} 4454 0
useless prefetch more than a thousand bytes away from the action
 (p17)	p16,p0=r32,r37				//3: {938:2} 4464 0
compare whether r32 (next value of i) < nNodes
 (p17)	add	r34=r35,r33				//3: {938:2} 4461 0
set r34 (next value of r35) to r35 + size field, or so it hopes
 (p16)	br.wtop.dptk	.b4_110 ;;			//3: {938:2} 4465 0
rename the registers and do it again, if the cmp4 returned true

The problem with this code is that r27, which ought to be always equal
to r35 + 6, is incremented by the wrong amount in the second ld2
instruction, namely by the "i" counter.  The value that *should* get
added to it is the node size field, ie the same value that's loaded into
r33 below that and then added to r35 in the last add instruction (and
then stored into r34, which is about to become r35).  So I think the
compiler has outsmarted itself as to which rotating register contains
which value when.

The result of this breakage is that the set of node pointers computed by
the loop is totally wrong for all values after the first.  This means
the later loop that's trying to insert the now-known downlink TIDs into
the innerTuple's nodes is storing those TIDs into random locations, and
thus tromping all over memory.  The case where we get a reproducible
crash is where the Asserts in that loop notice that what's at the
pointed-to addresses isn't what's expected, before we manage to clobber
anything critical.

Diagnosis: icc 11.1 is not ready for prime time.

I shall now retire with a glass of wine and attempt to forget everything
I just learned about IA64.  What a bizarre architecture ...

			regards, tom lane

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