[x86/retpoline] Split the LLVM concept of retpolines into separate subtarget features for indirect calls and indirect branches.

[chandlerc]

Summary

This is in preparation for enabling *only* the call retpolines when
using speculative load hardening.

I've continued to use subtarget features for now as they continue to
seem the best fit given the lack of other retpoline like constructs so
far.

The LLVM side is pretty simple. I'd like to eventually get rid of the
old feature, but not sure what backwards compatibility issues that will
cause.

This does remove the "implies" from requesting an external thunk. This
always seemed somewhat questionable and is now clearly not desirable --
you specify a thunk the same way no matter which set of things are
getting retpolines.

I really want to keep this nicely isolated from end users and just an
LLVM implementation detail, so I've moved the -mretpoline flag in
Clang to no longer rely on a specific subtarget feature by that name and
instead to be directly handled. In some ways this is simpler, but in
order to preserve existing behavior I've had to add some fallback code
so that users who relied on merely passing -mretpoline-external-thunk
continue to get the same behavior. We should eventually remove this
I suspect (we have never tested that it works!) but I've not done that
in this patch.

[echristo]

LGTM. Thanks.

[chandlerc]

Thanks!

I'm going to go ahead and land this, but happy to iterate on anything if others have comments.

include/clang/Driver/Options.td

 def mno_pascal_strings : Flag<["-"], "mno-pascal-strings">,
   Alias<fno_pascal_strings>;
 def mno_red_zone : Flag<["-"], "mno-red-zone">, Group<m_Group>;
 def mno_relax_all : Flag<["-"], "mno-relax-all">, Group<m_Group>;
 def mno_rtd: Flag<["-"], "mno-rtd">, Group<m_Group>;
 def mno_soft_float : Flag<["-"], "mno-soft-float">, Group<m_Group>;
 def mno_stackrealign : Flag<["-"], "mno-stackrealign">, Group<m_Group>;
 
+def mretpoline : Flag<["-"], "mretpoline">, Group<m_Group>, Flags<[CoreOption,DriverOption]>;
+def mno_retpoline : Flag<["-"], "mno-retpoline">, Group<m_Group>, Flags<[CoreOption,DriverOption]>;
+
 def mrelax : Flag<["-"], "mrelax">, Group<m_riscv_Features_Group>,
   HelpText<"Enable linker relaxation">;
 def mno_relax : Flag<["-"], "mno-relax">, Group<m_riscv_Features_Group>,
   HelpText<"Disable linker relaxation">;
 
 def munaligned_access : Flag<["-"], "munaligned-access">, Group<m_arm_Features_Group>,
   HelpText<"Allow memory accesses to be unaligned (AArch32/AArch64 only)">;
 def mno_unaligned_access : Flag<["-"], "mno-unaligned-access">, Group<m_arm_Features_Group>,

 def mxsavec : Flag<["-"], "mxsavec">, Group<m_x86_Features_Group>;
 def mno_xsavec : Flag<["-"], "mno-xsavec">, Group<m_x86_Features_Group>;
 def mxsaveopt : Flag<["-"], "mxsaveopt">, Group<m_x86_Features_Group>;
 def mno_xsaveopt : Flag<["-"], "mno-xsaveopt">, Group<m_x86_Features_Group>;
 def mxsaves : Flag<["-"], "mxsaves">, Group<m_x86_Features_Group>;
 def mno_xsaves : Flag<["-"], "mno-xsaves">, Group<m_x86_Features_Group>;
 def mshstk : Flag<["-"], "mshstk">, Group<m_x86_Features_Group>;
 def mno_shstk : Flag<["-"], "mno-shstk">, Group<m_x86_Features_Group>;
-def mretpoline : Flag<["-"], "mretpoline">, Group<m_x86_Features_Group>;
-def mno_retpoline : Flag<["-"], "mno-retpoline">, Group<m_x86_Features_Group>;
 def mretpoline_external_thunk : Flag<["-"], "mretpoline-external-thunk">, Group<m_x86_Features_Group>;
 def mno_retpoline_external_thunk : Flag<["-"], "mno-retpoline-external-thunk">, Group<m_x86_Features_Group>;
 
 // These are legacy user-facing driver-level option spellings. They are always
 // aliases for options that are spelled using the more common Unix / GNU flag
 // style of double-dash and equals-joined flags.
 def gcc_toolchain_legacy_spelling : Separate<["-"], "gcc-toolchain">, Alias<gcc_toolchain>;
 def target_legacy_spelling : Separate<["-"], "target">, Alias<target>;

lib/Basic/Targets/X86.h

class LLVM_LIBRARY_VISIBILITY X86TargetInfo : public TargetInfo {
   bool HasCLDEMOTE = false;
   bool HasPCONFIG = false;
   bool HasPKU = false;
   bool HasCLFLUSHOPT = false;
   bool HasCLWB = false;
   bool HasMOVBE = false;
   bool HasPREFETCHWT1 = false;
   bool HasRDPID = false;
-  bool HasRetpoline = false;
   bool HasRetpolineExternalThunk = false;
   bool HasLAHFSAHF = false;
   bool HasWBNOINVD = false;
   bool HasWAITPKG = false;
   bool HasMOVDIRI = false;
   bool HasMOVDIR64B = false;
   bool HasPTWRITE = false;
   bool HasINVPCID = false;

lib/Basic/Targets/X86.cpp

for (const auto &Feature : Features) {
     } else if (Feature == "+prefetchwt1") {
       HasPREFETCHWT1 = true;
     } else if (Feature == "+clzero") {
       HasCLZERO = true;
     } else if (Feature == "+cldemote") {
       HasCLDEMOTE = true;
     } else if (Feature == "+rdpid") {
       HasRDPID = true;
-    } else if (Feature == "+retpoline") {
-      HasRetpoline = true;
     } else if (Feature == "+retpoline-external-thunk") {
       HasRetpolineExternalThunk = true;
     } else if (Feature == "+sahf") {
       HasLAHFSAHF = true;
     } else if (Feature == "+waitpkg") {
       HasWAITPKG = true;
     } else if (Feature == "+movdiri") {
       HasMOVDIRI = true;
return llvm::StringSwitch<bool>(Feature)
       .Case("pku", HasPKU)
       .Case("popcnt", HasPOPCNT)
       .Case("prefetchwt1", HasPREFETCHWT1)
       .Case("prfchw", HasPRFCHW)
       .Case("ptwrite", HasPTWRITE)
       .Case("rdpid", HasRDPID)
       .Case("rdrnd", HasRDRND)
       .Case("rdseed", HasRDSEED)
-      .Case("retpoline", HasRetpoline)
       .Case("retpoline-external-thunk", HasRetpolineExternalThunk)
       .Case("rtm", HasRTM)
       .Case("sahf", HasLAHFSAHF)
       .Case("sgx", HasSGX)
       .Case("sha", HasSHA)
       .Case("shstk", HasSHSTK)
       .Case("sse", SSELevel >= SSE1)
       .Case("sse2", SSELevel >= SSE2)

lib/Driver/ToolChains/Arch/X86.cpp

void x86::getX86TargetFeatures(const Driver &D, const llvm::Triple &Triple,
   if (Triple.isAndroid()) {
     if (ArchType == llvm::Triple::x86_64) {
       Features.push_back("+sse4.2");
       Features.push_back("+popcnt");
     } else
       Features.push_back("+ssse3");
   }
 
+  // Translate the high level `-mretpoline` flag to the specific target feature
+  // flags. We also detect if the user asked for retpoline external thunks but
+  // failed to ask for retpolines themselves. This is a bit hacky but keeps
+  // existing usages working. We should consider deprecated this and instead
+  // warning if the user requests external retpoline thunks and *doesn't*
+  // request some form of retpolines.
+  if (Args.hasArgNoClaim(options::OPT_mretpoline, options::OPT_mno_retpoline)) {
+    if (Args.hasFlag(options::OPT_mretpoline, options::OPT_mno_retpoline,
+                     false)) {
+      Features.push_back("+retpoline-indirect-calls");
+      Features.push_back("+retpoline-indirect-branches");
+    }
+  } else if (Args.hasFlag(options::OPT_mretpoline_external_thunk,
+                          options::OPT_mno_retpoline_external_thunk, false)) {
+    // FIXME: Add a warning about failing to specify `-mretpoline` and
+    // eventually switch to an error here.
+    Features.push_back("+retpoline-indirect-calls");
+    Features.push_back("+retpoline-indirect-branches");
+  }
+
   // Now add any that the user explicitly requested on the command line,
   // which may override the defaults.
   handleTargetFeaturesGroup(Args, Features, options::OPT_m_x86_Features_Group);
 }

test/Driver/x86-target-features.c

 
 // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mrdpid %s -### -o %t.o 2>&1 | FileCheck -check-prefix=RDPID %s
 // RUN: %clang -target i386-unknown-linux-gnu -march=i386 -mno-rdpid %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-RDPID %s
 // RDPID: "-target-feature" "+rdpid"
 // NO-RDPID: "-target-feature" "-rdpid"
 
 // RUN: %clang -target i386-linux-gnu -mretpoline %s -### -o %t.o 2>&1 | FileCheck -check-prefix=RETPOLINE %s
 // RUN: %clang -target i386-linux-gnu -mno-retpoline %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-RETPOLINE %s
-// RETPOLINE: "-target-feature" "+retpoline"
-// NO-RETPOLINE: "-target-feature" "-retpoline"
+// RETPOLINE: "-target-feature" "+retpoline-indirect-calls" "-target-feature" "+retpoline-indirect-branches"
+// NO-RETPOLINE-NOT: retpoline
 
 // RUN: %clang -target i386-linux-gnu -mretpoline -mretpoline-external-thunk %s -### -o %t.o 2>&1 | FileCheck -check-prefix=RETPOLINE-EXTERNAL-THUNK %s
 // RUN: %clang -target i386-linux-gnu -mretpoline -mno-retpoline-external-thunk %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-RETPOLINE-EXTERNAL-THUNK %s
 // RETPOLINE-EXTERNAL-THUNK: "-target-feature" "+retpoline-external-thunk"
 // NO-RETPOLINE-EXTERNAL-THUNK: "-target-feature" "-retpoline-external-thunk"
 
 // RUN: %clang -target i386-linux-gnu -mwaitpkg %s -### -o %t.o 2>&1 | FileCheck -check-prefix=WAITPKG %s
 // RUN: %clang -target i386-linux-gnu -mno-waitpkg %s -### -o %t.o 2>&1 | FileCheck -check-prefix=NO-WAITPKG %s