Add function attribute "patchable-function-prefix" to support -fpatchable-function-entry=N,M where M>0

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

(cherry picked from commit 22467e259507f5ead2a87d989251b4c951a587e4)

1 files changed, 40 insertions, 0 deletions

diff --git a/llvm/test/CodeGen/AArch64/patchable-function-entry.ll b/llvm/test/CodeGen/AArch64/patchable-function-entry.ll
index 6d84f23ba5e..5ae9d88a289 100644
--- a/llvm/test/CodeGen/AArch64/patchable-function-entry.ll
+++ b/llvm/test/CodeGen/AArch64/patchable-function-entry.ll
@@ -25,6 +25,10 @@ define i32 @f1() "patchable-function-entry"="1" {
   ret i32 0
 }
 
+;; Without -function-sections, f2 is in the same text section as f1.
+;; They share the __patchable_function_entries section.
+;; With -function-sections, f1 and f2 are in different text sections.
+;; Use separate __patchable_function_entries.
 define void @f2() "patchable-function-entry"="2" {
 ; CHECK-LABEL: f2:
 ; CHECK-NEXT: .Lfunc_begin2:
@@ -63,3 +67,39 @@ define void @f5() "patchable-function-entry"="5" comdat {
   %frame = alloca i8, i32 16
   ret void
 }
+
+;; -fpatchable-function-entry=3,2
+;; "patchable-function-prefix" emits data before the function entry label.
+define void @f3_2() "patchable-function-entry"="1" "patchable-function-prefix"="2" {
+; CHECK-LABEL: .type f3_2,@function
+; CHECK-NEXT: .Ltmp1: // @f3_2
+; CHECK-NEXT:  nop
+; CHECK-NEXT:  nop
+; CHECK-NEXT: f3_2:
+; CHECK:      // %bb.0:
+; CHECK-NEXT:  nop
+; CHECK-NEXT:  ret
+;; .size does not include the prefix.
+; CHECK:      .Lfunc_end5:
+; CHECK-NEXT: .size f3_2, .Lfunc_end5-f3_2
+; NOFSECT     .section __patchable_function_entries,"awo",@progbits,f1,unique,0
+; FSECT:      .section __patchable_function_entries,"awo",@progbits,f3_2,unique,4
+; CHECK:      .p2align 3
+; CHECK-NEXT: .xword .Ltmp1
+  ret void
+}
+
+;; When prefix data is used, arbitrarily place NOPs after prefix data.
+define void @prefix() "patchable-function-entry"="0" "patchable-function-prefix"="1" prefix i32 1 {
+; CHECK-LABEL: .type prefix,@function
+; CHECK-NEXT: .word 1 // @prefix
+; CHECK:      .Ltmp2:
+; CHECK:       nop
+; CHECK-NEXT: prefix:
+;; Emit a __patchable_function_entries entry even if "patchable-function-entry" is 0.
+; NOFSECT     .section __patchable_function_entries,"awo",@progbits,prefix,unique,0
+; FSECT:      .section __patchable_function_entries,"awo",@progbits,prefix,unique,5
+; CHECK:      .p2align 3
+; CHECK-NEXT: .xword .Ltmp2
+  ret void
+}