Similar to the function attribute `prefix` (prefix data),
"patchable-function-prefix" inserts data (M NOPs) before the function
entry label.
-fpatchable-function-entry=2,1 (1 NOP before entry, 1 NOP after entry)
will look like:
```
.type foo,@function
.Ltmp0: # @foo
nop
foo:
.Lfunc_begin0:
# optional `bti c` (AArch64 Branch Target Identification) or
# `endbr64` (Intel Indirect Branch Tracking)
nop
.section __patchable_function_entries,"awo",@progbits,get,unique,0
.p2align 3
.quad .Ltmp0
```
-fpatchable-function-entry=N,0 + -mbranch-protection=bti/-fcf-protection=branch has two reasonable
placements (https://gcc.gnu.org/ml/gcc-patches/2020-01/msg01185.html):
```
(a) (b)
func: func:
.Ltmp0: bti c
bti c .Ltmp0:
nop nop
```
(a) needs no additional code. If the consensus is to go for (b), we will
need more code in AArch64BranchTargets.cpp / X86IndirectBranchTracking.cpp .
Differential Revision: https://reviews.llvm.org/D73070
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
Fangrui Song