[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, 11 insertions, 0 deletions

diff --git a/llvm/test/Verifier/invalid-patchable-function-entry.ll b/llvm/test/Verifier/invalid-patchable-function-entry.ll
new file mode 100644
index 00000000000..bd70ce0fc04
--- /dev/null
+++ b/llvm/test/Verifier/invalid-patchable-function-entry.ll
@@ -0,0 +1,11 @@
+; RUN: not llvm-as %s -o /dev/null 2>&1 | FileCheck %s
+
+; CHECK: "patchable-function-entry" takes an unsigned integer:
+; CHECK: "patchable-function-entry" takes an unsigned integer: a
+; CHECK: "patchable-function-entry" takes an unsigned integer: -1
+; CHECK: "patchable-function-entry" takes an unsigned integer: 3,
+
+define void @f() "patchable-function-entry" { ret void }
+define void @fa() "patchable-function-entry"="a" { ret void }
+define void @f_1() "patchable-function-entry"="-1" { ret void }
+define void @f3comma() "patchable-function-entry"="3," { ret void }