af126b8f0cf9ddc553a78869a3c820a4bd4fdc59
braney
  Mon Aug 17 13:36:13 2026 -0700
build: link the browser against zlib-ng instead of the system zlib, refs #38125

Adds zlib-ng as a second git submodule, alongside htslib, and points ZLIB at it.
It is built with --zlib-compat, so the header and the symbol names are the
ordinary zlib ones and no calling code has to change.

Measured over the eight Recommended Track Set pages, hgTracks uses 29 percent
less processor time and 26 percent less wall clock, and the image it produces is
pixel identical. Writing the PNG is about three times faster and reading a
bigBed data block about twice as fast, which is why the gain is larger than the
image work alone would give.

This also makes the platforms agree. x86_64 was linking a static /lib64/libz.a
only because that file happened to be present, the arm64 Docker build fell
through to a shared -lz, and Darwin was taking MacPorts zlib. All three now use
the submodule.

Verified on x86_64 Linux, aarch64 Linux and Apple silicon: zlib-ng builds, its
own test suite passes, the makefiles pick it up, and bigBedToBed output is byte
identical to before the change.

diff --git src/inc/common.mk src/inc/common.mk
index f16671ddcf0..0a78054e177 100644
--- src/inc/common.mk
+++ src/inc/common.mk
@@ -56,51 +56,54 @@
 endif
 L = ${STATIC_PRE}
 
 ifeq (${HOSTNAME},hgwdev)
   IS_HGWDEV = yes
   OURSTUFF = /cluster/software/r9
 else
   ifeq (${HOSTNAME},hgwdev-old.gi.ucsc.edu)
     IS_HGWDEV = yes
     OURSTUFF = /cluster/software
   else
      IS_HGWDEV = no
   endif
 endif
 
+# zlib comes from the zlib-ng submodule in submodules/zlib-ng, built in
+# zlib-compat mode so the header and the symbols are the ordinary zlib ones, and
+# linked statically the way htslib is.  zlib-ng writes a PNG about three times
+# faster and reads a bigBed block about twice as fast, and its output is ordinary
+# deflate that any zlib can read.  Building it here also makes the platforms
+# agree: before this, x86_64 happened to find a static /lib64/libz.a while the
+# arm64 build fell through to a shared -lz.  Override on the make command line
+# with ZLIB=... to link some other zlib.  refs #38125
 ifeq (${ZLIB},)
-  ifneq ($(wildcard /lib64/libz.a),)
-    ZLIB=/lib64/libz.a
-  else
-    ZLIB=-lz
-  endif
+  ZLIB=$(kentSrc)/submodules/zlib-ng/libz.a
 endif
 
 # for Darwin (Mac OSX), use static libs when they can be found
 ifeq ($(UNAME_S),Darwin)
   ifneq ($(wildcard /opt/local/include/openssl/ssl.h),)
     HG_INC += -I/opt/local/include
   endif
   # on M1, the directory changed
   ifneq ($(wildcard /opt/homebrew/include/openssl/ssl.h),)
     HG_INC += -I/opt/homebrew/include
     L += -L/opt/homebrew/lib/
   endif
-  ifneq ($(wildcard /opt/local/lib/libz.a),)
-    ZLIB = /opt/local/lib/libz.a
-  endif
+  # no ZLIB line here: Darwin uses the zlib-ng submodule like every other
+  # platform.  refs #38125
   ifneq ($(wildcard /opt/local/lib/libpng.a),)
     PNGLIB = /opt/local/lib/libpng.a
   endif
   ifeq (${BZ2LIB},)
     ifneq ($(wildcard /opt/local/lib/libbz2.a),)
       BZ2LIB=/opt/local/lib/libbz2.a
     endif
   endif
   ifneq ($(wildcard /opt/local/lib/libfreetype.a),)
     FREETYPELIBS = /opt/local/lib/libfreetype.a
   endif
   ifneq ($(wildcard /opt/local/lib/libbrotlidec.a),)
     FREETYPELIBS += /opt/local/lib/libbrotlidec.a /opt/local/lib/libbrotlicommon.a
   else
     ifneq ($(wildcard /usr/local/Cellar/brotli/1.1.0/lib/libbrotlidec.a),)