flambda-backend: Unconditionally reserve the top header byte in runtime 4 and runtime 5 (#2389)

Author: ncik-roberts

configure

@@ -61,7 +61,9 @@ CSCFLAGS=""
 ostype="Unix"
 SO="so"
 toolchain="cc"
-reserved_header_bits=0
+reserved_header_bits=8
+profinfo=true
+profinfo_width=8
 custom_ops_struct_size=64
 instrumented_runtime=false
 instrumented_runtime_libs=""
@@ -205,6 +207,8 @@ AC_SUBST([install_bytecode_programs])
 AC_SUBST([install_source_artifacts])
 AC_SUBST([install_ocamlnat])
 AC_SUBST([reserved_header_bits])
+AC_SUBST([profinfo])
+AC_SUBST([profinfo_width])
 AC_SUBST([custom_ops_struct_size])
 AC_SUBST([frame_pointers])
 AC_SUBST([cpp_mangling])
@@ -463,14 +467,6 @@ AC_ARG_WITH([target-bindir],
   [AS_HELP_STRING([--with-target-bindir],
     [location of binary programs on target system])])
 
-AC_ARG_ENABLE([reserved-header-bits],
-  [AS_HELP_STRING([--enable-reserved-header-bits=BITS],
-    [reserve BITS (between 0 and 31) bits in block headers])],
-  [AS_CASE([$enable_reserved_header_bits],
-    [[[0-9]]|[[1-2]][[0-9]]|3[[0-1]]],
-      [ reserved_header_bits=$enable_reserved_header_bits],
-  [AC_MSG_ERROR([invalid argument to --enable-reserved-header-bits])])])
-
 AC_ARG_ENABLE([stdlib-manpages],
   [AS_HELP_STRING([--disable-stdlib-manpages],
     [do not build or install the library man pages])])
@@ -2207,7 +2203,12 @@ AS_IF([test x"$enable_naked_pointers_checker" = "xyes" ],
 ## Check for mmap support for huge pages and contiguous heap
 OCAML_MMAP_SUPPORTS_HUGE_PAGES
 
+# Unconditionally reserve 8 header bits in both runtime 4 and
+# runtime 5, which is required for mixed block support.
 AC_DEFINE_UNQUOTED([HEADER_RESERVED_BITS], [$reserved_header_bits])
+AC_DEFINE_UNQUOTED([PROFINFO_WIDTH], [$profinfo_width])
+AS_IF([$profinfo], [AC_DEFINE([WITH_PROFINFO])])
+
 AC_DEFINE_UNQUOTED([CUSTOM_OPS_STRUCT_SIZE], [$custom_ops_struct_size])
 
 AS_IF([test x"$enable_installing_bytecode_programs" = "xno"],

configure.ac

@@ -105,8 +105,10 @@ For 64-bit architectures:
      +----------+--------+-------+-----+
 bits  63    64-R 63-R  10 9     8 7   0
 
-where 0 <= R <= 31 is HEADER_RESERVED_BITS, set with the
---enable-reserved-header-bits=R argument to configure.
+where 0 <= R <= 31 is HEADER_RESERVED_BITS. R is always
+set to 8 for the flambda-backend compiler in order to support
+mixed blocks. In the upstream compiler, R is set with the
+--enable-reserved-header-bits=R argument.
 
 */
 

runtime/caml/mlvalues.h

@@ -103,14 +103,15 @@ For 64-bit architectures:
      +--------+-------+-----+
 bits  63    10 9     8 7   0
 
-For x86-64 with Spacetime profiling:
-  P = PROFINFO_WIDTH (as set by "configure", currently 26 bits, giving a
-    maximum block size of just under 4Gb)
+For 64-bit architectures with mixed block support enabled:
+  P = PROFINFO_WIDTH (as set by "configure", currently 8 bits)
      +----------------+----------------+-------------+
-     | profiling info | wosize         | color | tag |
+     | scannable size | wosize         | color | tag |
      +----------------+----------------+-------------+
 bits  63        (64-P) (63-P)        10 9     8 7   0
 
+Mixed block support uses the PROFINFO_WIDTH functionality
+originally built for Spacetime profiling, hence the odd name.
 */
 
 #define Tag_hd(hd) ((tag_t) ((hd) & 0xFF))

runtime4/caml/mlvalues.h

@@ -39,6 +39,6 @@ ${main} -conf --disable-native-compiler \
         -no-native
 ${main} -conf --disable-flat-float-array
 ${main} -conf --enable-flambda
-${main} -conf --enable-frame-pointers -conf --enable-reserved-header-bits=27
+${main} -conf --enable-frame-pointers
 ${main} -with-bootstrap -conf --with-pic
 OCAMLRUNPARAM="c=1" ${main}

tools/ci/inria/other-configs/script

@@ -99,7 +99,6 @@ let asm = {@QS@|@AS@|@QS@}
 let asm_cfi_supported = @asm_cfi_supported@
 let with_frame_pointers = @frame_pointers@
 let with_cpp_mangling = @cpp_mangling@
-let reserved_header_bits = @reserved_header_bits@
 let custom_ops_struct_size = @custom_ops_struct_size@
 
 let ext_exe = {@QS@|@exeext@|@QS@}
@@ -119,3 +118,6 @@ let ar_supports_response_files = @ar_supports_response_files@
 
 let naked_pointers = "@naked_pointers@" = "true"
 let runtime5 = "@enable_runtime5@" = "yes"
+
+let reserved_header_bits =
+  if runtime5 then @reserved_header_bits@ else @profinfo_width@

utils/config.generated.ml.in

@@ -233,7 +233,14 @@ val with_cmm_invariants : bool
 (** Whether the invariants checks for Cmm are enabled *)
 
 val reserved_header_bits : int
-(** How many bits of a block's header are reserved *)
+(** How many bits of a block's header are reserved. This is correct
+   regardless of whether we're in runtime 4 or runtime 5.
+
+   In runtime 5, this corresponds to the HEADER_RESERVED_BITS C preprocessor
+   macro. In runtime 4, this corresponds to the PROFINFO_WIDTH C preprocessor
+   macro. Both of these are unconditionally set to a constant by the configure
+   script in order to enable mixed block support.
+ *)
 
 val custom_ops_struct_size : int
 (** Size in bytes of the custom operations structure. *)