topic "What is BLITZ and how are precompiled headers supported";
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[{_}%EN-US 
[s2; About BLITZ and precompiled headers&]
[s0; [* Blitz] is advanced compilation technique based on [^http`:`/`/en`.wikipedia`.org`/wiki`/Single`_Compilation`_Unit^ S
CU] approach, intended to speedup debug mode rebuilds of large 
applications. In fact, BLITZ is what allows U`+`+ to keep libraries 
in sources form. You can consider BLITZ as an automated form 
SCU.&]
[s0; &]
[s0; Blitz processes packages (not the whole program) `- each package 
can have a single [/ blitz`-block] (blitz block  is SCU).&]
[s0; &]
[s0; Only .cpp including files with inclusion guards (#ifdef NAME`_H 
#define NAME`_H .... #endif) can qualify to be part of [/ blitz`-block]. 
&]
[s0; &]
[s0; Alternatively, you can force inclusion by&]
[s7; &]
[s7; #pragma BLITZ`_APPROVE&]
[s7; &]
[s0; or&]
[s7; &]
[s7; //#BLITZ`_APPROVE&]
[s7; &]
[s0; &]
[s0; (also works for header) or exclusion by&]
[s0; &]
[s7; #pragma BLITZ`_PROHIBIT.&]
[s7; &]
[s0; or&]
[s7; &]
[s7; //#BLITZ`_PROHIBIT&]
[s7; &]
[s0; &]
[s0; In addition only files older than one hour qualify for [/ blitz`-block]. 
This simple heuristics excludes file that are being worked on 
`- otherwise change in file would always require rebuilding of 
the whole package, which would be potentially slower.&]
[s0; &]
[s0; The files in package that do not qualify for [/ blitz`-block ]are 
build the regular way.&]
[s0; &]
[s0; It is also possible to disable blitz for individual packages, 
either on project basis (in Package organizer) or in Output mode 
dialog (machine specific setting).&]
[s0; &]
[s0; Files in [/ blitz`-block] are scanned for any #define, these are 
undefined after the file. [/ blitz`-block] is in fact a file generated 
into output directory that include all blitz approved files and 
gets compiled instead of them (it is named `$blitz.cpp, you can 
check the output directory for details).&]
[s0; &]
[s0; BLITZ also includes one dirty trick to basically allow BLITZ 
compilation of packages that define static variables names based 
on the line number (macro `_`_LINE`_`_). Static variables are 
not visible outside the compilation unit using the normal build 
process, but when using BLITZ, more than single .cpp files are 
compiled into single unit and so there is the possibility that 
two such variables would have the same name.&]
[s0; &]
[s0; Therefore BLITZ adds BLITZ`_INDEX`_`_ macro, which contains 
a unique number (index of file) for each .cpp. That way you can 
create `"static`" identifiers e.g. like this (and also make it 
compile without BLITZ):&]
[s0; &]
[ {{10000@(255.255.192) [s7; #define MK`_`_s`_`_(x)       s`_`_s##x&]
[s7; #define MK`_`_s`_(x)        MK`_`_s`_`_(x)&]
[s7; &]
[s7; #ifdef  BLITZ`_INDEX`_`_&]
[s7; #define STATIC`_ID            MK`_`_s`_(COMBINE(BLITZ`_INDEX`_`_, 
`_`_LINE`_`_))&]
[s7; #else&]
[s7; #define STATIC`_ID            MK`_`_s`_(`_`_LINE`_`_)&]
[s0; [C@5;1 -|#endif]]}}&]
[s0; &]
[s0; [*^http`:`/`/en`.wikipedia`.org`/wiki`/Precompiled`_header^ Precompiled 
headers] is a compiler technique trying to solve the very same 
problem. In general, we have found BLITZ faster than any precompiled 
header use, however BLITZ tends to have one disadvantage: by combining 
all files into single object file, linker has less opportunity 
to remove unused code. This leads to (sometimes significantly) 
larger executable binaries. For this reason, we do not recommend 
(and have off by default) BLITZ for release builds and if possible, 
we use precompiled headers for release builds.&]
[s0; &]
[s0; Precompiled headers have a set of its own problems. Notably, 
Microsoft C`+`+ precompiled headers are hard to use with multiple 
processes building the code (Hydra) in debug mode. Also, precompiled 
headers in general are very bulky files, easily surpassing 100MB, 
which is a problem as we need to have single precompiled header 
per package.&]
[s0; &]
[s0; For these reasons precompiled headers support works like this:&]
[s0; &]
[s0;i150;O2; Precompiled headers are activated only in release mode 
without blitz.&]
[s0;i150;O2; You have to set `"Precompile header`" flag on header 
files candidates for precompilation. Only single candidate per 
package is allowed. Note that not all headers can be with this 
system. Header has to have include guards and it must be possible 
for all files in the package to include it first before all other 
headers.&]
[s0;i150;O2; Build method has to have `"Allow precompiled headers`" 
set.&]
[s0;i150;O2; When package is build, it first checks whether using 
precompiled header is possible (as per rules above). Then it 
checks how many files are to be rebuild. If there are 3 or more 
files to build, U`+`+ precompiles the header and uses it to build 
the package. When the package is built, U`+`+ deletes the header 
to conserve the space.&]
[s0; &]
[s0; U`+`+ supports precompiled headers for MSC, GCC and CLANG. However, 
practical benchmarks show that with CLANG using precompiled headers 
actually leads to worse compilation times.&]
[s0; ]]