[x86/SLH] Add a real Clang flag and LLVM IR attribute for Speculative

Load Hardening.

Wires up the existing pass to work with a proper IR attribute rather
than just a hidden/internal flag. The internal flag continues to work
for now, but I'll likely remove it soon.

Most of the churn here is adding the IR attribute. I talked about this
Kristof Beyls and he seemed at least initially OK with this direction.
The idea of using a full attribute here is that we *do* expect at least
some forms of this for other architectures. There isn't anything
*inherently* x86-specific about this technique, just that we only have
an implementation for x86 at the moment.

While we could potentially expose this as a Clang-level attribute as
well, that seems like a good question to defer for the moment as it
isn't 100% clear whether that or some other programmer interface (or
both?) would be best. We'll defer the programmer interface side of this
for now, but at least get to the point where the feature can be enabled
without relying on implementation details.

This also allows us to do something that was really hard before: we can
enable *just* the indirect call retpolines when using SLH. For x86, we
don't have any other way to mitigate indirect calls. Other architectures
may take a different approach of course, and none of this is surfaced to
user-level flags.

Differential Revision: https://reviews.llvm.org/D51157

llvm-svn: 341363

4 files changed, 21 insertions, 5 deletions

diff --git a/clang/lib/CodeGen/CGCall.cpp b/clang/lib/CodeGen/CGCall.cpp
index c2b4c95ae9e..8857ffdde49 100644
--- a/clang/lib/CodeGen/CGCall.cpp
+++ b/clang/lib/CodeGen/CGCall.cpp
@@ -1784,6 +1784,9 @@ void CodeGenModule::ConstructDefaultFnAttrList(StringRef Name, bool HasOptnone,
       FuncAttrs.addAttribute("stackrealign");
     if (CodeGenOpts.Backchain)
       FuncAttrs.addAttribute("backchain");
+
+    if (CodeGenOpts.SpeculativeLoadHardening)
+      FuncAttrs.addAttribute(llvm::Attribute::SpeculativeLoadHardening);
   }
 
   if (getLangOpts().assumeFunctionsAreConvergent()) {
diff --git a/clang/lib/Driver/ToolChains/Arch/X86.cpp b/clang/lib/Driver/ToolChains/Arch/X86.cpp
index ff2023c2cbe..bc707857d8f 100644
--- a/clang/lib/Driver/ToolChains/Arch/X86.cpp
+++ b/clang/lib/Driver/ToolChains/Arch/X86.cpp
@@ -146,15 +146,23 @@ void x86::getX86TargetFeatures(const Driver &D, const llvm::Triple &Triple,
 
   // 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)) {
+  // failed to ask for retpolines themselves (through any of the different
+  // flags). This is a bit hacky but keeps existing usages working. We should
+  // consider deprecating this and instead warn 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,
+                         options::OPT_mspeculative_load_hardening,
+                         options::OPT_mno_speculative_load_hardening)) {
     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_mspeculative_load_hardening,
+                            options::OPT_mno_speculative_load_hardening,
+                            false)) {
+      // On x86, speculative load hardening relies on at least using retpolines
+      // for indirect calls.
+      Features.push_back("+retpoline-indirect-calls");
     }
   } else if (Args.hasFlag(options::OPT_mretpoline_external_thunk,
                           options::OPT_mno_retpoline_external_thunk, false)) {
diff --git a/clang/lib/Driver/ToolChains/Clang.cpp b/clang/lib/Driver/ToolChains/Clang.cpp
index ff083c79f10..79129a7a7c6 100644
--- a/clang/lib/Driver/ToolChains/Clang.cpp
+++ b/clang/lib/Driver/ToolChains/Clang.cpp
@@ -4132,6 +4132,9 @@ void Clang::ConstructJob(Compilation &C, const JobAction &JA,
 
   Args.AddLastArg(CmdArgs, options::OPT_pthread);
 
+  Args.AddLastArg(CmdArgs, options::OPT_mspeculative_load_hardening,
+                  options::OPT_mno_speculative_load_hardening);
+
   RenderSSPOptions(getToolChain(), Args, CmdArgs, KernelOrKext);
 
   // Translate -mstackrealign
diff --git a/clang/lib/Frontend/CompilerInvocation.cpp b/clang/lib/Frontend/CompilerInvocation.cpp
index 4ac79293282..5210314493d 100644
--- a/clang/lib/Frontend/CompilerInvocation.cpp
+++ b/clang/lib/Frontend/CompilerInvocation.cpp
@@ -1147,6 +1147,8 @@ static bool ParseCodeGenArgs(CodeGenOptions &Opts, ArgList &Args, InputKind IK,
 
   Opts.KeepStaticConsts = Args.hasArg(OPT_fkeep_static_consts);
 
+  Opts.SpeculativeLoadHardening = Args.hasArg(OPT_mspeculative_load_hardening);
+
   return Success;
 }