Dear Thomas,
thank you for your input. I've debugged one of the packages and I
confirm that the breakage is related to passing of strings from C to
Fortran. Indeed, BLAS and LAPACK define a large number of subroutines
that take one or more explicit single-character strings as arguments.
Other than that, BLAS has only one function (xerbla), which takes a
string of unspecified length, LAPACK only has four (ilaenv,
ilaenv2stage, lsamen, xerbla). The C interfaces to BLAS/LAPACK from
Netlib depend on the historic behavior that explicit single-character
strings are interoperable, concretely CBLAS and LAPACKE provide C
interfaces/code that calls into Fortran BLAS/LAPACK without passing the
1s as lengths of the character strings (functions taking a string of
unspecified length are trivial and re-implemented in C). This has been
working fine for very many years as the Fortran code never needed to
access the length it knew was 1. R has been using the same practice,
which long predates ISO_C_BINDING/BIND(C), and I've seen online
discussions where people assumed interoperability of length 1 strings,
once mentioning also a citation from Fortran 2003 Handbook that says "A
Fortran character string with a length other than 1 is not
interoperable" (which invites interpretation that length 1 strings were
). I am not an expert to say whether the current Fortran standard
requires that interoperability and I assume that it does not given this
gfortran change.
This gfortran change breaks this interoperability: if a C function calls
a Fortran function, passing it a single-character string for a parameter
taking explicit single-character Fortran string, it may crash. I've
debugged one case with R package BDgraph, this example
"library(BDgraph); data.sim <- bdgraph.sim( n = 70, p = 5, size = 7, vis
= TRUE )" crashes due to corruption of C stack by Fortran function
DPOSV, when compiled with the new gfortran and with -O2. To see the
problem, one can just look at the disassembly of DPOSV (LAPACK), neither
the package nor R is not necessary:
SUBROUTINE DPOSV( UPLO, N, NRHS, A, LDA, B, LDB, INFO )
CHARACTER UPLO
In one case, DPOSV calls DPOTRS before returning. The new gfortran with
-O2 performs tail-call optimization, jumping to DPOTRS. In the annotated
disassembly snippet, at 11747f1, DPOSV tries to ensure that there is
constant 1 as string length of UPLO when tail-calling into DPOTRS, so it
writes it to stack where there already should have been 1 as length of
UPLO passed to DPOSV. But this argument has not been passed to DPOSV, so
this causes stack corruption.
1174ce: 0f 85 62 ff ff ff jne 117436 <dposv_+0xb6> <== jump if
ERROR
CALL DPOTRS( UPLO, N, NRHS, A, LDA, B, LDB, INFO )
1174d4: 48 8b 04 24 mov (%rsp),%rax <======= rax holds
LDB
1174d8: 4c 89 7c 24 68 mov %r15,0x68(%rsp) <=== save INFO
to output param
1174dd: 49 89 d8 mov %rbx,%r8 <========== pass LDA
as LDA
1174e0: 4c 89 e1 mov %r12,%rcx <========= pass A as A
1174e3: 4c 8b 4c 24 08 mov 0x8(%rsp),%r9 <===== pass B as B
1174e8: 4c 89 ea mov %r13,%rdx <========= pass NRHS
as NRHS
1174eb: 48 89 ee mov %rbp,%rsi <========= pass N as N
1174ee: 4c 89 f7 mov %r14,%rdi <========= pass UPLO
as UPLO
1174f1: 48 c7 44 24 70 01 00 movq $0x1,0x70(%rsp) <=== pass 1
hidden arg on stack CORRUPTS C STACK
1174f8: 00 00
1174fa: 48 89 44 24 60 mov %rax,0x60(%rsp) <=== pass LDB
as LDB (stack)
END
1174ff: 48 83 c4 28 add $0x28,%rsp <== remove 5 vars
from stack (sframe)
117503: 5b pop %rbx
117504: 5d pop %rbp
117505: 41 5c pop %r12
117507: 41 5d pop %r13
117509: 41 5e pop %r14
11750b: 41 5f pop %r15 <=== restore register to
level before call
CALL DPOTRS( UPLO, N, NRHS, A, LDA, B, LDB, INFO )
11750d: e9 de 56 ef ff jmpq cbf0 <dpotrs_@plt> <=== tail
call to dpotrs
Note that DPOSV never uses the length of the string (UPLO) from the
hidden argument, the compiler clearly knows that its length is 1. In
calls where the length is passed in registers, this does not cause
trouble (like LSAME) and indeed is needed as the registers have
different values
IF( .NOT.LSAME( UPLO, 'U' ) .AND. .NOT.LSAME( UPLO, 'L' ) ) THEN
117448: b9 01 00 00 00 mov $0x1,%ecx
11744d: ba 01 00 00 00 mov $0x1,%edx
117452: 48 8d 35 bb 12 09 00 lea 0x912bb(%rip),%rsi #
1a8714 <ipivot.4261+0xd14>
117459: 4c 89 f7 mov %r14,%rdi
11745c: e8 1f 3d ef ff callq b180 <lsame_@plt>
but it seems to me that the compiler could just refrain from setting the
length to be 1 on the stack at 1174f1, since it knows it should have
already been there. It would be a nice property if Fortran code that
never accesses the hidden arguments with the lengths of the strings,
because it knows what those lengths are, would also never write to those
hidden arguments on the stack when it knows what they are (should be).
Before the gfortran change, DPOSV would call to DPOTRS normally (no
tail-call optimization), so this problem would not occur (I tested with
268974). By disabling tail call optimization via
-fno-optimize-sibling-calls, the problem goes away also for other
packages my colleagues have identified as crashing with the new
gfortran. Did you know of any other optimization that could break this
interoperability of 1-length strings? It would be really nice to users
if this interoperability could be preserved, and if not by default than
at least with some option.
Traditionally, BLAS/LAPACK implementations are interchangeable at
dynamic linking time, using the Fortran interface that is however also
used from C, without passing lengths for fixed 1-character strings. R
supports this too, at least on some Linux distributions including
Debian/Ubuntu it is packaged so that it runs with the BLAS/LAPACK
implementation installed on the system. Even though this is probably not
correct wrt to the todays Fortran standard (I don't know for sure), this
is the common practice, and fixing this would not be easy - one would
have to create a new interface to be used from C, separate from the
Fortran one, and all software would have to start using that interface
from C. In the current situation when the Fortran interface is used,
confusion will arise with this gfortran change as different BLAS/LAPACK
implementations are built by different Fortran compilers and use a
different mix of Fortran/C for different computational subroutines. Note
CBLAS could not be readily used as it itself breaks with the current
gfortran change as well.
The same interoperability considerations apply to R packages, which
include native code that calls from C or from Fortran into the (same)
Fortran interface of BLAS/LAPACK. There would have to be a commonly
accepted C interface instead by the BLAS/LAPACK implementations, and all
of these packages would have to be modified to use that interface. If we
created such a C interface just inside R and asked all package
maintainers to update their packages, we would still have the problem
with substitution of external BLAS(/LAPACK) implementations at dynamic
linking time.
Indeed, it would be very hard to identify these problems by testing,
because at least now the crashes are quite rate (for the tail-call
optimization, a number of conditions have to be met to cause memory
corruption, first the tail optimization has to happen, then the number
of arguments has to be so large (on x86) that the lengths are passed on
stack and not in registers, we have to be lucky for the memory
corruption to map to a crash, etc).
So, any help we could get from you would be highly appreciated, be it
just a compile option to keep the old behavior or an assurance that we
are fine if we just disable the tail-call optimization. Appreciated by
us but I believe also many others who use or develop BLAS/LAPACK, but
may not have yet run into the problem, as they may not have been
regularly testing bleeding-edge versions of compilers or may not have
such a large code base to test as we have on CRAN.
Thanks
Tomas
On 4/24/19 11:32 PM, Thomas König wrote:
> Hi,
>
> I have tried to pinpoint potential problems which could lead to the
> LAPACK issues that are currently seen in R. I built the current R
> trunk using
>
> AR=gcc-ar RANLIB=gcc-ranlib ./configure --prefix=$HOME --enable-lto
> --enable-BLAS-shlib=no --without-recommended-packages
>
> and used this to find problem areas.
>
> There are quite a few warnings that were flagged, due to mismatches
> in function types.
>
> The prototypes that R has in its header files, for example BLAS.h,
> are often not compatible with gfortran function declarations. To take
> one small example, in src/main/print.c, we have
>
> void NORET F77_NAME(xerbla)(const char *srname, int *info)
>
> so xerbla_ is defined with two arguments.
>
> However, gfortran passes string lengths as hidden arguments.
> You can see this by compiling the small example
>
> $ cat xer.f
> SUBROUTINE FOO
> INTEGER INFO
> CALL XERBLA ('FOO', INFO)
> END
> $ gfortran -c -fdump-tree-original xer.f
> $ cat xer.f.004t.original
> foo ()
> {
> integer(kind=4) info;
>
> xerbla (&"FOO"[1]{lb: 1 sz: 1}, &info, 3);
> }
>
> so here we have three arguments. This mismatch is flagged
> by -Wlto-type-mismatch, which, for example, yields
>
> print.c:1120:12: note: type 'void' should match type 'long int'
> ../../src/extra/blas/blas.f:357:20: warning: type of 'xerbla' does not
> match original declaration [-Wlto-type-mismatch]
> 357 | CALL XERBLA( 'DGBMV ', INFO )
>
>
> So, why can gcc's r268992 / r269349 matter? Before these patches,
> gfortran used the variadic calling convention for calling procedures
> outside the current file, and the non-variadic calling convention for
> calling procedures found in the current file.
>
> Because the procedures were all compiled as non-variadic, the caller and
> the calle's signature did not match if they were not in the same
> source file, which is an ABI violation.
>
> This violation manifested itself in https://gcc.gnu.org/PR87689 ,
> where the the problem resulted in crashes on a primary gcc platform,
> POWER.
>
> How can this potentially affect R? After the fix for PR87689,
> gfortran's calls to external procedures are no longer variadic. It is
> quite possible that, while this "works" most of the time, there
> is a problem with a particular LAPACK routine, the call sequence
> leading up to it or the procedures it calls.
>
> How to fix this problem? The only clear way I see is to fix this
> on the R side, by adding the string lengths to the prototypes.
> These are size_t (64 bit on 64-bit systems, 32 bit on 32-bit
> systems). You should then try to make --enable-lto pass
> without any warnings.
>
> Regarding LAPACK itself, the default build system for R builds
> it as a shared library. Offhand, I did not see any way to
> build a *.a file instead, so I could not use LTO to check
> for mismatched prototypes between R and LAPACK.
>
> Of course, I cannot be sure that this is really the root cause
> of the problem you are seeing,but it does seem to fit quite well.
> I hope this analysis helps in resolving this.
>
> Regards
>
> Thomas
>
> ______________________________________________
> R-devel@r-project.org mailing list
> https://stat.ethz.ch/mailman/listinfo/r-devel
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