[llvm-bugs] [Bug 41215] New: Invalid checksum computation when compiling from preprocessed output

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=41215

            Bug ID: 41215
           Summary: Invalid checksum computation when compiling from
                    preprocessed output
           Product: clang
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: enhancement
          Priority: P
         Component: -New Bugs
          Assignee: unassignedclangbugs at nondot.org
          Reporter: lambert.clara at yahoo.fr
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org,
                    neeilans at live.com, richard-llvm at metafoo.co.uk

Created attachment 21662
  --> https://bugs.llvm.org/attachment.cgi?id=21662&action=edit
Fix

When clang-cl compiles an object from preprocessed output, the checksum of the
individual files that were included in it will be incorrect in the generated
debug info, since it will be the checksum of the preprocessed file, instead of
being the one from the files themselves.

This leads to an error when debugging from visual studio, since the files will
be considered different from their source, we get a popup with the message "The
source file is different from when the module was built. Would you like the
debugger to use it anyway?".

This can be reproduced with those steps:
- Create two files, main.cpp and lib.h
- Include lib.h in main.cpp
- Generate the pp output with `clang-cl /c main.cpp /Z7 /P"
- Compile the generated pp output to an obj
- Link and generate an exe and pdb
- Use `cvdump -sf main.pdb`
- Notice that under the main.obj module, each file's MD5 is the same:

** Module: "E:\RD\tests\clang_hash\main.obj"

     0 E:\RD\tests\clang_hash\lib.h (MD5: 136293700AE501A1FB76EBD273C8D288)
     1 C:\Program Files (x86)\Windows Kits\10\Include\10.0.17763.0\ucrt\stdio.h
(MD5: 136293700AE501A1FB76EBD273C8D288)
     2 E:\RD\tests\clang_hash\main.cpp (MD5: 136293700AE501A1FB76EBD273C8D288)
     3 C:\Program Files (x86)\Windows
Kits\10\Include\10.0.17763.0\ucrt\corecrt_stdio_config.h (MD5:
136293700AE501A1FB76EBD273C8D288)




MSVC compiler clears out the checksum field in that instance:

** Module: "E:\RD\tests\clang_hash\main.obj"

     0 c:\program files (x86)\windows kits\10\include\10.0.17763.0\ucrt\stdio.h
(None)
     1 c:\program files (x86)\windows
kits\10\include\10.0.17763.0\ucrt\corecrt_wstdio.h (None)
     2 c:\program files (x86)\windows
kits\10\include\10.0.17763.0\ucrt\corecrt_stdio_config.h (None)
     3 c:\program files (x86)\microsoft visual
studio\2017\professional\vc\tools\msvc\14.16.27023\include\vcruntime_new.h
(None)
     4 e:\rd\tests\clang_hash\main.cpp (None)
     5 e:\rd\tests\clang_hash\lib.h (None)

I've attached a patch for that issue, that makes the behavior of clang match
the one from MSVC.

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