[AArch64] Add function attribute "patchable-function-entry" to add NOPs at function entry

The Linux kernel uses -fpatchable-function-entry to implement DYNAMIC_FTRACE_WITH_REGS
for arm64 and parisc. GCC 8 implemented
-fpatchable-function-entry, which can be seen as a generalized form of
-mnop-mcount. The N,M form (function entry points before the Mth NOP) is
currently only used by parisc.

This patch adds N,0 support to AArch64 codegen. N is represented as the
function attribute "patchable-function-entry". We will use a different
function attribute for M, if we decide to implement it.

The patch reuses the existing patchable-function pass, and
TargetOpcode::PATCHABLE_FUNCTION_ENTER which is currently used by XRay.

When the integrated assembler is used, __patchable_function_entries will
be created for each text section with the SHF_LINK_ORDER flag to prevent
--gc-sections (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93197) and
COMDAT (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=93195) issues.

Retrospectively, __patchable_function_entries should use a PC-relative
relocation type to avoid the SHF_WRITE flag and dynamic relocations.

"patchable-function-entry"'s interaction with Branch Target
Identification is still unclear (see
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92424 for GCC discussions).

Reviewed By: peter.smith

Differential Revision: https://reviews.llvm.org/D72215

1 files changed, 12 insertions, 0 deletions

diff --git a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
index 2026bcce3e8..e062ca77e71 100644
--- a/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
+++ b/llvm/lib/Target/AArch64/AArch64AsmPrinter.cpp
@@ -243,6 +243,18 @@ void AArch64AsmPrinter::EmitStartOfAsmFile(Module &M) {
 
 void AArch64AsmPrinter::LowerPATCHABLE_FUNCTION_ENTER(const MachineInstr &MI)
 {
+  const Function &F = MF->getFunction();
+  if (F.hasFnAttribute("patchable-function-entry")) {
+    unsigned Num;
+    if (F.getFnAttribute("patchable-function-entry")
+            .getValueAsString()
+            .getAsInteger(10, Num))
+      return;
+    for (; Num; --Num)
+      EmitToStreamer(*OutStreamer, MCInstBuilder(AArch64::HINT).addImm(0));
+    return;
+  }
+
   EmitSled(MI, SledKind::FUNCTION_ENTER);
 }