[ELF] Optimization to LinkerScript::computeInputSections NFC

[andrewng]

Summary

Moved the section name check ahead of any filename matching or
exclusion. Firstly, this reduces the need to retrieve the filename and
secondly, reduces the amount of potentially expensive filename pattern
matching if such rules are present in the linker script.

The impact of this change is particularly significant when linking
objects built with -ffunction-sections and -fstack-size-section, using a
linker script that includes non-trivial filename patterns. In a number
of such cases, the link time is halved.

[MaskRay]

lld/ELF/LinkerScript.cpp

429

check the section name

[MaskRay]

It'd be great if you can provide the exact numbers.

[andrewng]

In D72775#1822384, @MaskRay wrote:

It'd be great if you can provide the exact numbers.

	No stack sizes	Stack Sizes	Overhead of stack sizes
Link 1	1.246s	2.127s	70.71%
Link 1*	0.874s	0.995s	13.84%
Link 2	2.042s	3.885s	90.25%
Link 2*	1.635s	1.933s	18.23%

Where * = with this patch
Run with a release build with assertions enabled on Windows 10 with AMD Ryzen 3900X (12C/24T).

lld/ELF/LinkerScript.cpp

for (InputSectionBase *sec : inputSections) {
       //   .rela.dyn : { *(.rela.data) }
       // which are common because they are in the default bfd script.
       // We do not ignore SHT_REL[A] linker-synthesized sections here because
       // want to support scripts that do custom layout for them.
       if (isa<InputSection>(sec) &&
           cast<InputSection>(sec)->getRelocatedSection())
         continue;
 
+      // Check the name early to improve performance in the common case.
+      if (!pat.sectionPat.match(sec->name))
+        continue;
+
       std::string filename = getFilename(sec->file);
-      if (!cmd->filePat.match(filename) ||
-          pat.excludedFilePat.match(filename) ||
-          !pat.sectionPat.match(sec->name))
+      if (!cmd->filePat.match(filename) || pat.excludedFilePat.match(filename))
         continue;
 
       ret.push_back(sec);
     }
 
     sortInputSections(
         MutableArrayRef<InputSectionBase *>(ret).slice(sizeBefore), pat);
   }