[LLD] [MinGW] Look for libs named "<libname>.lib" even with -static

[mstorsjo]

Summary

This matches how ld.bfd works in practice; this fixes
https://github.com/mstorsjo/llvm-mingw/issues/305.

Adding a test for the new lib name combination that this allows, but
also adding a few negative tests for combinations that aren't
matched when -static is specified (because this change in itself
didn't break any of the existing tests either).

The logic in how ld.bfd looks for various libraries based on
an -l<libname> argument is rather complex; the
ldemul_open_dynamic_archive function looks for various combinations:
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/emultempl/pep.em;h=e68d1e69f17ad73af065b6bed19ae89ded913172;hb=b51c2fec1da205ea3e7354cbb3e253018d64873c#l2066
This function is only called if looking for dynamic libraries
(i.e. if -static wasn't specified):
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/ldfile.c;h=731ae5f7aedcf921bd36a1b32a3e0f5bfa189071;hb=b51c2fec1da205ea3e7354cbb3e253018d64873c#l365

However even this function is skipped, it still looks for libraries
in the form of "lib<libname>.a" (this is what lld did before):
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/ldfile.c;h=731ae5f7aedcf921bd36a1b32a3e0f5bfa189071;hb=b51c2fec1da205ea3e7354cbb3e253018d64873c#l440
But it also calls a format specific function called
ldemul_find_potential_libraries, which for PE targets looks for
files named "<libname>.lib":
https://sourceware.org/git/?p=binutils-gdb.git;a=blob;f=ld/emultempl/pep.em;h=e68d1e69f17ad73af065b6bed19ae89ded913172;hb=b51c2fec1da205ea3e7354cbb3e253018d64873c#l2175

[alvinhochun]

Makes sense to me (though I haven't checked the logic of ld.bfd myself).

[mati865]

It's unfortunate that MSVC doesn't distinguish static from import libs by extension but we have to adjust.

[mstorsjo]

In D135651#3850183, @mati865 wrote:

It's unfortunate that MSVC doesn't distinguish static from import libs by extension but we have to adjust.

Some projects use the convention of naming import libs <name>.lib and static libs lib<name>.lib for MSVC style builds (e.g. the libc++ build does that). But in any case, regardless of what those libs are, we kinda need to follow what ld.bfd does here, for the least amount of surprises for end users.

lld/MinGW/Driver.cpp

for (StringRef dir : searchPaths) {
     if (!bStatic) {
       if (Optional<std::string> s = findFile(dir, "lib" + name + ".dll.a"))
         return *s;
       if (Optional<std::string> s = findFile(dir, name + ".dll.a"))
         return *s;
     }
     if (Optional<std::string> s = findFile(dir, "lib" + name + ".a"))
       return *s;
-    if (!bStatic) {
-      if (Optional<std::string> s = findFile(dir, name + ".lib"))
-        return *s;
+    if (!bStatic) {
       if (Optional<std::string> s = findFile(dir, "lib" + name + ".dll"))
         return *s;
       if (Optional<std::string> s = findFile(dir, name + ".dll"))
         return *s;
     }
   }
   error("unable to find library -l" + name);
   return "";

lld/test/MinGW/lib.test

 LIB5-SAME: libbar.dll.a
 
 RUN: echo > %t/lib/noprefix.dll.a
 RUN: echo > %t/lib/msvcstyle.lib
 RUN: ld.lld -### -m i386pep -L%t/lib -lnoprefix -lmsvcstyle 2>&1 | FileCheck -check-prefix=OTHERSTYLES %s
 OTHERSTYLES: noprefix.dll.a
 OTHERSTYLES-SAME: msvcstyle.lib
 
+RUN: not ld.lld -### -m i386pep -L%t/lib -static -lnoprefix 2>&1 | FileCheck -check-prefix=ERROR-NOPREFIX %s
+ERROR-NOPREFIX: unable to find library -lnoprefix
+
+RUN: ld.lld -### -m i386pep -L%t/lib -static -lmsvcstyle 2>&1 | FileCheck -check-prefix=MSVCSTYLE %s
+MSVCSTYLE: msvcstyle.lib
+
 RUN: echo > %t/lib/libnoimplib.dll
 RUN: echo > %t/lib/noprefix_noimplib.dll
 RUN: ld.lld -### -m i386pep -L%t/lib -lnoimplib 2>&1 | FileCheck -check-prefix=DLL1 %s
 RUN: ld.lld -### -m i386pep -L%t/lib -lnoprefix_noimplib 2>&1 | FileCheck -check-prefix=DLL2 %s
 DLL1: libnoimplib.dll
 DLL2: noprefix_noimplib.dll
+
+RUN: not ld.lld -### -m i386pep -L%t/lib -static -lnoimplib 2>&1 | FileCheck -check-prefix=ERROR-NOIMPLIB %s
+RUN: not ld.lld -### -m i386pep -L%t/lib -static -lnoprefix_noimplib 2>&1 | FileCheck -check-prefix=ERROR-NOPREFIX-NOIMPLIB %s
+ERROR-NOIMPLIB: unable to find library -lnoimplib
+ERROR-NOPREFIX-NOIMPLIB: unable to find library -lnoprefix_noimplib