Visual Studio 2019 always rebuilds everything after I hit the "build" button - c++

Question

I'm new to Visual Studio. Now I'm using Visual Studio 2019 to develop Clang&LLVM. I found that every time I right-click the clang executable in the Solution Explorer and choose "Build" or "Debug->Start new instance", VS starts to re-build everything from scratch, although I haven't changed any source file. Since LLVM is a giant project, this recompilation takes too long.
Is there any solution to make VS smarter to only recompile necessary files (files got changes and their dependencies)?

Answer 1

This could happen if there is a file with "copy always" ("copy to output directory" option)
Fix: Change it to "copy if newer"
You could find these easily by checking the (.vcxproj) project file in a text editor. (search for <CopyToOutputDirectory>, possible values: Never, Always, PreserveNewest. See msbuild-docs)

Answer 2

In my case this problem occur in one specific situation, when project is compile with flag /MP and have "precompiled header".
In above case while compiling your app the file "Microsoft.Build.CPPTasks.CL.read.1.tlog" is created and unfortunately contains paths to temporary files e.g
"\?\C:_Sources\MyApp\OBJ\DEBUG\X64\MyApp\stdafxD-8811C005.PCH.TMP
\?\C:_Sources\MyApp\OBJ\DEBUG\X64\MyApp\PCH-92DA10DF.OBJ.TMP"
When you run compilation again every entries from the file is checked for any changes, since we have entry points to not existing file, the compiler have to compile it again, we can see the "missing" file is a precompiled header, so the entire application must be recompiled
For now I found 3 solutions for this problem:
Do not use flag /MP
Use other build system like ninja, fastbuild, etc.
Write simple app to remove temporary entries from all *.tlog files
and run it before every build
Sample code in c# for solution 3.
static void Main(string[] args)
{
string sobjPath = #"C:\_Sources\MyApp\Obj";
string[] allfiles = Directory.GetFiles(sobjPath, "*.tlog", SearchOption.AllDirectories);
foreach(string filePath in allfiles)
{
List<string> lines = File.ReadAllLines(filePath).ToList();
var linesCount = lines.Count;
lines.RemoveAll(x => x.StartsWith(#"\\?") && x.EndsWith(#".TMP"));
if(linesCount != lines.Count)
{
File.WriteAllLines(filePath, lines);
}
}
}

Answer 3

Source of recompilation can be found in output if diagnostic is set to Tools->Options->Projects and Solutions->Build and Run->MSBuild project build output verbosity = Diagnostic.
In my case it was because they were first builds after installation of new computer. I checked out files from svn but later I changed computer date and time because it was incorrect. After change all previously checked files has date in future so recompilation occurred. To solve issue I deleted all sources and checked out again from svn.