bug#35205: [PATCH] Document IBM Z environment variables

[Ilya Leoshkevich]

IBM Z patch (7a6f9c9) introduced 5 environment variables, which are
used to tune the compression and decompression with the DEFLATE
COMPRESSION CALL instruction.  This patch adds texinfo documentation for
all of them.

* doc/gzip.texi(Environment): Add DFLTCC* environment variables.
---
 doc/gzip.texi | 38 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 37 insertions(+), 1 deletion(-)

diff --git a/doc/gzip.texi b/doc/gzip.texi
index a6d9896..77b3a56 100644
--- a/doc/gzip.texi
+++ b/doc/gzip.texi
@@ -69,7 +69,7 @@ Free Documentation License''.
 * Sample::             Sample output from @command{gzip}.
 * Invoking gzip::      How to run @command{gzip}.
 * Advanced usage::     Concatenated files.
-* Environment::                The @env{GZIP} environment variable
+* Environment::                Environment variables.
 * Tapes::               Using @command{gzip} on tapes.
 * Problems::           Reporting bugs.
 * GNU Free Documentation License:: Copying and sharing this manual.
@@ -513,6 +513,42 @@ export PATH=/usr/bin
 exec gzip -9 "$@@"
 @end example
 
+The following environment variables are applicable only when using
address@hidden on IBM Z mainframes supporting DEFLATE COMPRESSION CALL
+instruction:
+
address@hidden @option
+
address@hidden DFLTCC
+Whether DEFLATE COMPRESSION CALL should be used.  Default value is @samp{1}.
+Set this to @samp{0} to disable DEFLATE COMPRESSION CALL altogether.
+
address@hidden DFLTCC_LEVEL_MASK
+Compression levels on which DEFLATE COMPRESSION CALL should be used.
+Represented as a bit mask in decimal or hexadecimal form, where each bit
+corresponds to a compression level.  Default value is @samp{2}, which means
+level 1 only.  In order to make use of DEFLATE COMPRESSION CALL by default, 
that
+is, on levels 1-6, set this to @samp{0x7e}.
+
address@hidden DFLTCC_BLOCK_SIZE
+Size of deflate blocks produced by DEFLATE COMPRESSION CALL in bytes in decimal
+or hexadecimal form.  Default value is @samp{1048576} (1 megabyte).  When using
+DEFLATE COMPRESSION CALL to compress a file containing heterogeneous data (e.g.
+a @samp{.tar} archive containing text and binary files), setting this to a
+smaller value may improve compression ratio.
+
address@hidden DFLTCC_FIRST_FHT_BLOCK_SIZE
+Size of the first fixed deflate block produced by DEFLATE COMPRESSION CALL in
+bytes in decimal or hexadecimal form.  Default value is @samp{4096} (4
+kilobytes).  When using DEFLATE COMPRESSION CALL to compress a small file,
+setting this to a larger value may improve compression ratio.
+
address@hidden DFLTCC_RIBM
+Value of "Reserved for IBM" field of DEFLATE COMPRESSION CALL parameter block.
+Default value is @samp{0}.
+
address@hidden table
+
 @node Tapes
 @chapter Using @command{gzip} on tapes
 @cindex tapes
-- 
2.21.0