RFR: JDK-8151656: Minor tweaks to old Hotspot build to ease comparison with new

Erik Joelsson erik.joelsson at oracle.com
Wed Mar 16 08:51:11 UTC 2016

The __FILE__ macro is built into the compiler and evaluates to the file 
name of the current source file. It's intended use is for debug 
messages. In the rest of the JDK we eliminated its use completely by 
providing a macro called THIS_FILE that evaluates to the basename.ext of 
the source file being compiled. In Hotspot it's still used and it's used 
in header files, which makes the THIS_FILE solution invalid.

For most of our compilers, the format of the path is based on how the 
file was presented on the command line. This matters to us since the old 
Hotspot build uses relative paths to the generated source and header 
files and the new build uses absolute paths for all source files. Also, 
the directory structure in the output directory differ. This means that 
for certain object files, the constant strings resulting from the use of 
__FILE__ differ, both in contents and length. The difference in length 
causes alignment differences in the objects and the resulting libraries.

When creating the new build, we are trying very hard to make sure we are 
still building the same thing. One way to verify this is to compare 
various aspects of the built binaries. When possible, this is far easier 
than running a very large amount of tests and it gives us a large amount 
of confidence in our changes. The alignment differences caused by 
different lengths of constant strings severely limits the amount of 
clean comparisons we can do.

The particular fix here changes how we generate the files from the ADLC 
tool. The tool already generates #line directives which helps the 
compiler figure out which actual source file and line parts of the 
generated files came from. Basically overriding what __FILE__ and 
__LINE__ will evaluate to. Some of these lines need a bit of post 
processing, and for that, the build already uses awk to rewrite them. 
The current awk script looks for lines like this, which the ADLC tool 
prints when it doesn't know the correct source file:

#line 999999

and rewrites it to

#line <actual line number in generated file+1> <absolute path to 
generated file>

I changed this to instead rewrite to:

#line <actual line number in generated file> <basename.ext of generated 

I also added an initial #line first in each file.

#line 1 <basename.ext of generated file>

With these changes, we can do very clean comparisons on Linux, Solaris 
and Macosx. This helps the new hotspot build immensely, but will also be 
good in the future as we have a very convenient way of verifying build 
changes that shouldn't affect the built output.


On 2016-03-15 22:02, Christian Thalinger wrote:
>> On Mar 15, 2016, at 12:06 AM, Erik Joelsson <erik.joelsson at oracle.com> wrote:
>> Any chance I could get a second review on this? Preferably from the hotspot team.
> It’s not quite obvious to me what the __FILE__ change is.  Maybe an example would help.
>> /Erik
>> On 2016-03-11 18:16, Erik Joelsson wrote:
>>> In preparation for the new Hotspot build, there are a few changes to the old build that needs to happen first. These changes should be harmless, but do impact the generated binaries some. These changes are:
>>> * Standardizing sort order for objects on link command line on Windows to a locale independent order.
>>> * Working around compare differences caused by the __FILE__ macro in certain generated files by overriding the value of __FILE__ in those files.
>>> By making these changes first and separate from introducing the new build we reduce the potential impact of when we do introduce the new build.
>>> Bug: https://bugs.openjdk.java.net/browse/JDK-8151656
>>> Webrev: http://cr.openjdk.java.net/~erikj/8151656/index.html
>>> /Erik

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