[clang][ARM] PACBTI-M frontend support

Handle branch protection option on the commandline as well as a function
attribute. One patch for both mechanisms, as they use the same underlying
parsing mechanism.

These are recorded in a set of LLVM IR module-level attributes like we do for
AArch64 PAC/BTI (see https://reviews.llvm.org/D85649):

- command-line options are "translated" to module-level LLVM IR
  attributes (metadata).

- functions have PAC/BTI specific attributes iff the
  __attribute__((target("branch-protection=...))) was used in the function
  declaration.

- command-line option -mbranch-protection to armclang targeting Arm,
following this grammar:

branch-protection ::= "-mbranch-protection=" <protection>
protection ::=  "none" | "standard" | "bti" [ "+" <pac-ret-clause> ]
                | <pac-ret-clause> [ "+" "bti"]
pac-ret-clause ::= "pac-ret" [ "+" <pac-ret-option> ]
pac-ret-option ::= "leaf" ["+" "b-key"] | "b-key" ["+" "leaf"]

b-key is simply a placeholder to make it consistent with AArch64's
version. In Arm, however, it triggers a warning informing that b-key is
unsupported and a-key will be selected instead.

- Handle _attribute_((target(("branch-protection=..."))) for AArch32 with the
same grammer as the commandline options.

This patch is part of a series that adds support for the PACBTI-M extension of
the Armv8.1-M architecture, as detailed here:

https://community.arm.com/arm-community-blogs/b/architectures-and-processors-blog/posts/armv8-1-m-pointer-authentication-and-branch-target-identification-extension

The PACBTI-M specification can be found in the Armv8-M Architecture Reference
Manual:

https://developer.arm.com/documentation/ddi0553/latest

The following people contributed to this patch:

- Momchil Velikov
- Victor Campos
- Ties Stuij

Reviewed By: vhscampos

Differential Revision: https://reviews.llvm.org/D112421

1 files changed, 48 insertions, 0 deletions

diff --git a/llvm/lib/Support/TargetParser.cpp b/llvm/lib/Support/TargetParser.cpp
index 1dadce4..4acc23d 100644
--- a/llvm/lib/Support/TargetParser.cpp
+++ b/llvm/lib/Support/TargetParser.cpp
@@ -333,3 +333,51 @@ bool getCPUFeaturesExceptStdExt(CPUKind Kind,
 
 } // namespace RISCV
 } // namespace llvm
+
+// Parse a branch protection specification, which has the form
+//   standard | none | [bti,pac-ret[+b-key,+leaf]*]
+// Returns true on success, with individual elements of the specification
+// returned in `PBP`. Returns false in error, with `Err` containing
+// an erroneous part of the spec.
+bool ARM::parseBranchProtection(StringRef Spec, ParsedBranchProtection &PBP,
+                                StringRef &Err) {
+  PBP = {"none", "a_key", false};
+  if (Spec == "none")
+    return true; // defaults are ok
+
+  if (Spec == "standard") {
+    PBP.Scope = "non-leaf";
+    PBP.BranchTargetEnforcement = true;
+    return true;
+  }
+
+  SmallVector<StringRef, 4> Opts;
+  Spec.split(Opts, "+");
+  for (int I = 0, E = Opts.size(); I != E; ++I) {
+    StringRef Opt = Opts[I].trim();
+    if (Opt == "bti") {
+      PBP.BranchTargetEnforcement = true;
+      continue;
+    }
+    if (Opt == "pac-ret") {
+      PBP.Scope = "non-leaf";
+      for (; I + 1 != E; ++I) {
+        StringRef PACOpt = Opts[I + 1].trim();
+        if (PACOpt == "leaf")
+          PBP.Scope = "all";
+        else if (PACOpt == "b-key")
+          PBP.Key = "b_key";
+        else
+          break;
+      }
+      continue;
+    }
+    if (Opt == "")
+      Err = "<empty>";
+    else
+      Err = Opt;
+    return false;
+  }
+
+  return true;
+}