PZUNMR2(l)

📦 version 1.5, in ubuntu jammy

PZUNMR2(l) 💅✨

okay bestie, welcome to the PZUNMR2 page — it's a LAPACK routine and it's version 1.5, so it's giving vintage but iconic. buckle up, we're doing linear algebra 💖

🌸 NAME 🌸

PZUNMR2 overwrites the general complex M-by-N distributed matrix sub( C ) = C(IC:IC+M-1,JC:JC+N-1) with SIDE = 'L', SIDE = 'R', TRANS = 'N'.

🎀 SYNOPSIS 🎀

SUBROUTINE PZUNMR2( SIDE, TRANS, M, N, K, A, IA, JA, DESCA, TAU, C, IC,
                    JC, DESCC, WORK, LWORK, INFO )

    CHARACTER       SIDE, TRANS

    INTEGER         IA, IC, INFO, JA, JC, K, LWORK, M, N

    INTEGER         DESCA( * ), DESCC( * )

    COMPLEX*16      A( * ), C( * ), TAU( * ), WORK( * )

🦋 PURPOSE 🦋

so PZUNMR2 overwrites the general complex M-by-N distributed matrix sub( C ) = C(IC:IC+M-1,JC:JC+N-1) with

TRANS = 'C':      Q**H * sub( C )
                  sub( C ) * Q**H

where Q is a complex unitary distributed matrix defined as the product of K elementary reflectors

Q = H(1)' H(2)' . . . H(k)'

as returned by PZGERQF. Q is of order M if SIDE = 'L' and of order N if SIDE = 'R'. no cap, that's the whole vibe.

✨ Notes ✨

okay so real talk — every global data object has an associated description vector. that vector stores literally everything you need to map an object element to its corresponding process and memory location. it's giving ✨*coordinates*✨.

let A be a generic term for any 2D block cyclicly distributed array. such a global array has an associated description vector DESCA. in the following comments, the character _ should be read as "of the global array". iykyk.

  • DTYPE_A (global) — stored in DESCA( DTYPE_ ) — the descriptor type. in this case, DTYPE_A = 1.
  • CTXT_A (global) — stored in DESCA( CTXT_ ) — the BLACS context handle, indicating the BLACS process grid A is distributed over. the context itself is global, but the handle (the integer value) may vary.
  • M_A (global) — stored in DESCA( M_ ) — the number of rows in the global array A.
  • N_A (global) — stored in DESCA( N_ ) — the number of columns in the global array A.
  • MB_A (global) — stored in DESCA( MB_ ) — the blocking factor used to distribute the rows of the array.
  • NB_A (global) — stored in DESCA( NB_ ) — the blocking factor used to distribute the columns of the array.
  • RSRC_A (global) — stored in DESCA( RSRC_ ) — the process row over which the first row of the array A is distributed.
  • CSRC_A (global) — stored in DESCA( CSRC_ ) — the process column over which the first column of the array A is distributed.
  • LLD_A (local) — stored in DESCA( LLD_ ) — the leading dimension of the local array. LLD_A >= MAX(1,LOCr(M_A)).

now let K be the number of rows or columns of a distributed matrix, and assume its process grid has dimension p x q.

LOCr( K ) denotes the number of elements of K that a process would receive if K were distributed over the p processes of its process column.

similarly, LOCc( K ) denotes the number of elements of K that a process would receive if K were distributed over the q processes of its process row.

the values of LOCr() and LOCc() may be determined via a call to the ScaLAPACK tool function, NUMROC:

LOCr( M ) = NUMROC( M, MB_A, MYROW, RSRC_A, NPROW ),
LOCc( N ) = NUMROC( N, NB_A, MYCOL, CSRC_A, NPCOL ).

an upper bound for these quantities may be computed by:

LOCr( M ) <= ceil( ceil(M/MB_A)/NPROW )*MB_A
LOCc( N ) <= ceil( ceil(N/NB_A)/NPCOL )*NB_A

👑 ARGUMENTS 👑

  • SIDE (global input) CHARACTER — = 'L': apply Q or Q**H from the Left; = 'R': apply Q or Q**H from the Right. pick a side bestie, left or right 💅
  • TRANS (global input) CHARACTER — = 'N': No transpose, apply Q; = 'C': Conjugate transpose, apply Q**H.
  • M (global input) INTEGER — the number of rows to be operated on, i.e. the number of rows of the distributed submatrix sub( C ). M >= 0.
  • N (global input) INTEGER — the number of columns to be operated on, i.e. the number of columns of the distributed submatrix sub( C ). N >= 0.
  • K (global input) INTEGER — the number of elementary reflectors whose product defines the matrix Q. If SIDE = 'L', M >= K >= 0; if SIDE = 'R', N >= K >= 0.
  • A (local input) COMPLEX*16 pointer into the local memory to an array of dimension (LLD_A,LOCc(JA+M-1)) if SIDE='L', and (LLD_A,LOCc(JA+N-1)) if SIDE='R', where LLD_A >= MAX(1,LOCr(IA+K-1)). On entry, the i-th row must contain the vector which defines the elementary reflector H(i), IA <= i <= IA+K-1, as returned by PZGERQF in the K rows of its distributed matrix argument A(IA:IA+K-1,JA:*). A(IA:IA+K-1,JA:*) is modified by the routine but restored on exit. she's a little chaotic but she puts herself back together, periodt.
  • IA (global input) INTEGER — the row index in the global array A indicating the first row of sub( A ).
  • JA (global input) INTEGER — the column index in the global array A indicating the first column of sub( A ).
  • DESCA (global and local input) INTEGER array of dimension DLEN_. the array descriptor for the distributed matrix A.
  • TAU (local input) COMPLEX*16, array, dimension LOCc(IA+K-1). this array contains the scalar factors TAU(i) of the elementary reflectors H(i) as returned by PZGERQF. TAU is tied to the distributed matrix A. they're linked, fr fr.
  • C (local input/local output) COMPLEX*16 pointer into the local memory to an array of dimension (LLD_C,LOCc(JC+N-1)). on entry, the local pieces of the distributed matrix sub(C). on exit, sub( C ) is overwritten by Q*sub( C ) or Q'*sub( C ) or sub( C )*Q' or sub( C )*Q. she ate and left no crumbs, but like, the crumbs are the old values. gone. 💀
  • IC (global input) INTEGER — the row index in the global array C indicating the first row of sub( C ).
  • JC (global input) INTEGER — the column index in the global array C indicating the first column of sub( C ).
  • DESCC (global and local input) INTEGER array of dimension DLEN_. the array descriptor for the distributed matrix C.
  • WORK (local workspace/local output) COMPLEX*16 array, dimension (LWORK). on exit, WORK(1) returns the minimal and optimal LWORK. ✨receipts✨
  • LWORK (local or global input) INTEGER — the dimension of the array WORK. LWORK is local input and must be at least:

If SIDE = 'L',

LWORK >= MpC0 + MAX( MAX( 1, NqC0 ),
                     NUMROC( NUMROC( M+IROFFC,MB_A,0,0,NPROW ),
                             MB_A,0,0,LCMP ) );

if SIDE = 'R',

LWORK >= NqC0 + MAX( 1, MpC0 );

where

LCMP = LCM / NPROW with LCM = ICLM( NPROW, NPCOL ),

IROFFC = MOD( IC-1, MB_C ), ICOFFC = MOD( JC-1, NB_C ),
ICROW = INDXG2P( IC, MB_C, MYROW, RSRC_C, NPROW ),
ICCOL = INDXG2P( JC, NB_C, MYCOL, CSRC_C, NPCOL ),
MpC0 = NUMROC( M+IROFFC, MB_C, MYROW, ICROW, NPROW ),
NqC0 = NUMROC( N+ICOFFC, NB_C, MYCOL, ICCOL, NPCOL ),

ILCM, INDXG2P and NUMROC are ScaLAPACK tool functions; MYROW, MYCOL, NPROW and NPCOL can be determined by calling the subroutine BLACS_GRIDINFO.

If LWORK = -1, then LWORK is global input and a workspace query is assumed; the routine only calculates the minimum and optimal size for all work arrays. each of these values is returned in the first entry of the corresponding work array, and no error message is issued by PXERBLA. so like, she's not gonna yell at you for the query. she's chill like that. 🫶

  • INFO (local output) INTEGER —
    • = 0: successful exit 🎉
    • < 0: if the i-th argument is an array and the j-entry had an illegal value, then INFO = -(i*100+j); if the i-th argument is a scalar and had an illegal value, then INFO = -i.

💀 Alignment requirements 💀

okay this part is the strict part, bestie. the distributed submatrices A(IA:*, JA:*) and C(IC:IC+M-1,JC:JC+N-1) must verify some alignment properties, namely the following expressions should be true:

If SIDE = 'L',

( NB_A.EQ.MB_C .AND. ICOFFA.EQ.IROFFC )

If SIDE = 'R',

( NB_A.EQ.NB_C .AND. ICOFFA.EQ.ICOFFC .AND. IACOL.EQ.ICCOL )
ngl if these are false you are in your flop era. check your descriptors. 🙈

and that's the whole page, diva ✨

this was PZUNMR2 from LAPACK version 1.5, dated 12 May 1997 — yeah she's older than most of us but she's still iconic, still running, still slaying 💖🎀🦋🌸💅👑🌈⭐🍓🧚💜

📦 where this page comes from