[Clang] Accept absolute paths in the -fuse-ld option

[whitequark]

Summary

This patch extends the -fuse-ld option to accept a full path to an executable
and use it verbatim to invoke the linker. There are generally two reasons
to desire this.

The first reason relates to the sad truth is that Clang is retargetable,
Binutils are not.

While any Clang from a binary distribution is sufficient to compile code
for a wide range of architectures and prefixed BFD linkers (e.g.
installed as /usr/bin/arm-none-linux-gnueabi-ld) as well as cross-compiled
libc's (for non-bare-metal targets) are widely available, including on all
Debian derivatives, it is impossible to use them together because
the -fuse-ld= option allows to specify neither a linker prefix nor
a full path to one.

The second reason is linker development, both when porting existing linkers
to new architectures and when working on a new linker such as LLD.

[silvas]

Some nits but this looks fine to me. I've been wanting this for a while now.

cfe/trunk/lib/Driver/ToolChain.cpp

347

Use llvm::sys::path::is_absolute so that this works on windows.

352

No need for else after return.

[whitequark]

cfe/trunk/lib/Driver/ToolChain.cpp

352

This would change behavior when an absolute path is passed but doesn't exist; clang will try to append it to "ld." and display some silly error message.

[whitequark]

Windows compatibility

[silvas]

cfe/trunk/lib/Driver/ToolChain.cpp

352

Makes sense.

[jroelofs]

Testcases?

[echristo]

Needs a testcase, but otherwise should be fine.

[whitequark]

Added a test

[echristo]

LGTM, thanks.

-eric

cfe/trunk/lib/Driver/ToolChain.cpp

 }
 
 std::string ToolChain::GetProgramPath(const char *Name) const {
   return D.GetProgramPath(Name, *this);
 }
 
 std::string ToolChain::GetLinkerPath() const {
   if (Arg *A = Args.getLastArg(options::OPT_fuse_ld_EQ)) {
-    StringRef Suffix = A->getValue();
+    StringRef UseLinker = A->getValue();
 
+    if (llvm::sys::path::is_absolute(UseLinker)) {
+      // If we're passed -fuse-ld= with what looks like an absolute path,
+      // don't attempt to second-guess that.
+      if (llvm::sys::fs::exists(UseLinker))
+        return UseLinker;
+    } else {
       // If we're passed -fuse-ld= with no argument, or with the argument ld,
       // then use whatever the default system linker is.
-    if (Suffix.empty() || Suffix == "ld")
+      if (UseLinker.empty() || UseLinker == "ld")
         return GetProgramPath("ld");
 
       llvm::SmallString<8> LinkerName("ld.");
-    LinkerName.append(Suffix);
+      LinkerName.append(UseLinker);
 
       std::string LinkerPath(GetProgramPath(LinkerName.c_str()));
       if (llvm::sys::fs::exists(LinkerPath))
         return LinkerPath;
+    }
 
     getDriver().Diag(diag::err_drv_invalid_linker_name) << A->getAsString(Args);
     return "";
   }
 
   return GetProgramPath(DefaultLinker);
 }
 

cfe/trunk/test/Driver/fuse-ld.c

 // RUN: %clang %s -### \
+// RUN:     -fuse-ld=/usr/local/bin/or1k-linux-ld 2>&1 \
+// RUN:   | FileCheck %s --check-prefix=CHECK-ABSOLUTE-LD
+// CHECK-ABSOLUTE-LD: /usr/local/bin/or1k-linux-ld
+
+
+// RUN: %clang %s -### \
 // RUN:     -target x86_64-unknown-freebsd 2>&1 \
 // RUN:   | FileCheck %s --check-prefix=CHECK-FREEBSD-LD
 // CHECK-FREEBSD-LD: ld
 
 // RUN: %clang %s -### -fuse-ld=bfd \
 // RUN:     --sysroot=%S/Inputs/basic_freebsd_tree \
 // RUN:     -target x86_64-unknown-freebsd \
 // RUN:     -B%S/Inputs/basic_freebsd_tree/usr/bin 2>&1 \