[AArch64][ELF] Add support for PLT decoding with BTI instructions present

Arm Architecture v8.5a introduces Branch Target Identification (BTI). When
enabled all indirect branches must target a bti instruction of the
appropriate form. As PLT sequences may sometimes be the target of an
indirect branch and PLT[0] always is, a static linker may need to generate
PLT sequences that contain "bti c" as the first instruction. In effect:
bti     c
adrp    x16, page offset to .got.plt
...
Instead of:
adrp    x16, page offset to .got.plt
...
At present the PLT decoding assumes the adrp will always be the first
instruction. This patch adds support for a single "bti c" to prefix it. A
test binary has been uploaded with such a PLT sequence. A forthcoming LLD
patch will make heavy use of the PLT decoding code.

Differential Revision: https://reviews.llvm.org/D62598

llvm-svn: 362523

1 files changed, 9 insertions, 1 deletions

diff --git a/llvm/lib/Target/AArch64/MCTargetDesc/AArch64MCTargetDesc.cpp b/llvm/lib/Target/AArch64/MCTargetDesc/AArch64MCTargetDesc.cpp
index 26dd5e5adcc..df12274d947 100644
--- a/llvm/lib/Target/AArch64/MCTargetDesc/AArch64MCTargetDesc.cpp
+++ b/llvm/lib/Target/AArch64/MCTargetDesc/AArch64MCTargetDesc.cpp
@@ -333,12 +333,20 @@ public:
     for (uint64_t Byte = 0, End = PltContents.size(); Byte + 7 < End;
          Byte += 4) {
       uint32_t Insn = support::endian::read32le(PltContents.data() + Byte);
+      uint64_t Off = 0;
+      // Check for optional bti c that prefixes adrp in BTI enabled entries
+      if (Insn == 0xd503245f) {
+         Off = 4;
+         Insn = support::endian::read32le(PltContents.data() + Byte + Off);
+      }
       // Check for adrp.
       if ((Insn & 0x9f000000) != 0x90000000)
         continue;
+      Off += 4;
       uint64_t Imm = (((PltSectionVA + Byte) >> 12) << 12) +
             (((Insn >> 29) & 3) << 12) + (((Insn >> 5) & 0x3ffff) << 14);
-      uint32_t Insn2 = support::endian::read32le(PltContents.data() + Byte + 4);
+      uint32_t Insn2 =
+          support::endian::read32le(PltContents.data() + Byte + Off);
       // Check for: ldr Xt, [Xn, #pimm].
       if (Insn2 >> 22 == 0x3e5) {
         Imm += ((Insn2 >> 10) & 0xfff) << 3;