[PowerPC] Fix check for ieeelongdouble support

[nikic]

Summary

Clang detects the GCC version from the libdir. However, modern GCC versions only include the major version in the libdir (something like lib/gcc/powerpc64le-linux-gnu/12/), not all version components. For this reason, even though the system has a supported libstdcxx, it will still fail the check against the 12.1.0 version requirement.

Fix this by doing the same thing we do for patch versions: Assume that a missing minor version is larger than any specific version.

To allow this to be tested, we need to fix two additional issues: First, the GCC toolchain directories used for testing need to contain a crtbegin.o file to be properly detected. The existing tests actually ended up using a 0.0.0 version, rather the intended one. Second, we also need to satisfy the glibc version check based on the dynamic linker. To do so, respect the --dyld-prefix argument and add the necessary file to the test toolchain directory.

[nikic]

I also have related question here: The compatibility check currently unconditionally rejects libcxx. Do you have any additional information in what way libcxx is incompatible with the ieeelongdouble ABI?

[tbaeder]

The changes make sense to me but I'd like someone else to approve as well.

[nemanjai]

In D136258#3870429, @nikic wrote:

I also have related question here: The compatibility check currently unconditionally rejects libcxx. Do you have any additional information in what way libcxx is incompatible with the ieeelongdouble ABI?

PowerPC has two incompatible ABI's for long double. The underlying type is either "PPC double-double" (i.e. two double values in two separate registers) or IEEE 128-bit quad precision. The latter resides in a 128-bt vector register on PPC.

The changes here seem fine to me. Perhaps @ecnelises/@qiucf (not sure which ID is the right one) has further feedback here.

[qiucf]

Thanks for the fix! LGTM, sorry for the absense.

(but I remember GCC minor version always begins with 1?)

In D136258#3870429, @nikic wrote:

I also have related question here: The compatibility check currently unconditionally rejects libcxx. Do you have any additional information in what way libcxx is incompatible with the ieeelongdouble ABI?

Because currently libcxx doesn't support float128 (what ieeelongdouble abi emits) on PowerPC (and other targets, as I know). Once libcxx has the support, we can modify the check.

clang/lib/Driver/ToolChains/Gnu.cpp

 /// command line options.
 
 /// Less-than for GCCVersion, implementing a Strict Weak Ordering.
 bool Generic_GCC::GCCVersion::isOlderThan(int RHSMajor, int RHSMinor,
                                           int RHSPatch,
                                           StringRef RHSPatchSuffix) const {
   if (Major != RHSMajor)
     return Major < RHSMajor;
-  if (Minor != RHSMinor)
+  if (Minor != RHSMinor) {
+    // Note that versions without a specified minor sort higher than those with
+    // a minor.
+    if (RHSMinor == -1)
+      return true;
+    if (Minor == -1)
+      return false;
     return Minor < RHSMinor;
+  }
   if (Patch != RHSPatch) {
     // Note that versions without a specified patch sort higher than those with
     // a patch.
     if (RHSPatch == -1)
       return true;
     if (Patch == -1)
       return false;
 

clang/lib/Driver/ToolChains/PPCLinux.cpp

if (Args.hasArg(options::OPT_nostdlib, options::OPT_nostdlibxx))
     return true;
 
   CXXStdlibType StdLib = ToolChain::GetCXXStdlibType(Args);
   bool HasUnsupportedCXXLib =
       StdLib == CST_Libcxx ||
       (StdLib == CST_Libstdcxx &&
        GCCInstallation.getVersion().isOlderThan(12, 1, 0));
 
-  return GlibcSupportsFloat128(Linux::getDynamicLinker(Args)) &&
+  std::string Linker = Linux::getDynamicLinker(Args);
+  return GlibcSupportsFloat128((Twine(D.DyldPrefix) + Linker).str()) &&
          !(D.CCCIsCXX() && HasUnsupportedCXXLib);
 }

clang/test/Driver/ppc-float-abi-warning.cpp

 // REQUIRES: powerpc-registered-target
 // RUN: %clang -### --driver-mode=g++ -target powerpc64le-linux-gnu %s \
 // RUN:  --gcc-toolchain=%S/Inputs/powerpc64le-linux-gnu-tree/gcc-11.2.0 \
 // RUN:  -mabi=ieeelongdouble -stdlib=libstdc++ 2>&1 | FileCheck %s
 // RUN: %clang -### --driver-mode=g++ -target powerpc64le-linux-gnu %s \
+// RUN:  --gcc-toolchain=%S/Inputs/powerpc64le-linux-gnu-tree/gcc-12 \
+// RUN:  --dyld-prefix=%S/Inputs/powerpc64le-linux-gnu-tree/gcc-12 \
+// RUN:  -mabi=ieeelongdouble -stdlib=libstdc++ 2>&1 | \
+// RUN:  FileCheck %s --check-prefix=NOWARN
+// RUN: %clang -### --driver-mode=g++ -target powerpc64le-linux-gnu %s \
 // RUN:  -mabi=ieeelongdouble -stdlib=libc++ 2>&1 | FileCheck %s
 // RUN: %clang -### --driver-mode=g++ -target powerpc64le-linux-gnu %s\
 // RUN:  -mabi=ieeelongdouble -stdlib=libc++ -Wno-unsupported-abi 2>&1 | \
 // RUN:  FileCheck %s --check-prefix=NOWARN
 
 // CHECK: warning: float ABI 'ieeelongdouble' is not supported by current library
 // NOWARN-NOT: warning: float ABI 'ieeelongdouble' is not supported by current library
 long double foo(long double x) { return x; }