[llvm] [AArch64][PAC] Lower jump-tables using hardened pseudo. (PR #97666)

Ahmed Bougacha via llvm-commits llvm-commits at lists.llvm.org
Mon Jul 22 08:57:30 PDT 2024


================
@@ -1310,6 +1312,138 @@ void AArch64AsmPrinter::LowerJumpTableDest(llvm::MCStreamer &OutStreamer,
                                   .addImm(Size == 4 ? 0 : 2));
 }
 
+void AArch64AsmPrinter::LowerHardenedBRJumpTable(const MachineInstr &MI) {
+  unsigned InstsEmitted = 0;
+
+  const MachineJumpTableInfo *MJTI = MF->getJumpTableInfo();
+  assert(MJTI && "Can't lower jump-table dispatch without JTI");
+
+  const std::vector<MachineJumpTableEntry> &JTs = MJTI->getJumpTables();
+  assert(!JTs.empty() && "Invalid JT index for jump-table dispatch");
+
+  // Emit:
+  //     mov x17, #<size of table>     ; depending on table size, with MOVKs
+  //     cmp x16, x17                  ; or #imm if table size fits in 12-bit
+  //     csel x16, x16, xzr, ls        ; check for index overflow
+  //
+  //     adrp x17, Ltable at PAGE         ; materialize table address
+  //     add x17, Ltable at PAGEOFF
+  //     ldrsw x16, [x17, x16, lsl #2] ; load table entry
+  //
+  //   Lanchor:
+  //     adr x17, Lanchor              ; compute target address
+  //     add x16, x17, x16
+  //     br x16                        ; branch to target
+
+  MachineOperand JTOp = MI.getOperand(0);
+
+  unsigned JTI = JTOp.getIndex();
+  assert(!AArch64FI->getJumpTableEntryPCRelSymbol(JTI) &&
+         "unsupported compressed jump table");
+
+  const uint64_t NumTableEntries = JTs[JTI].MBBs.size();
+
+  // cmp only supports a 12-bit immediate.  If we need more, materialize the
+  // immediate, using x17 as a scratch register.
+  uint64_t MaxTableEntry = NumTableEntries - 1;
+  if (isUInt<12>(MaxTableEntry)) {
+    EmitToStreamer(*OutStreamer, MCInstBuilder(AArch64::SUBSXri)
+                                     .addReg(AArch64::XZR)
+                                     .addReg(AArch64::X16)
+                                     .addImm(MaxTableEntry)
+                                     .addImm(0));
+    ++InstsEmitted;
----------------
ahmedbougacha wrote:

Yes, let's do that for the whole file someplace else

https://github.com/llvm/llvm-project/pull/97666


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