From a8fbdc576990653e92ce1d766659005678fd8514 Mon Sep 17 00:00:00 2001
From: Fangrui Song <maskray@google.com>
Date: Sat, 4 Jan 2020 19:52:36 -0800
Subject: [PATCH] [X86] Support function attribute "patchable-function-entry"

For x86-64, we diverge from GCC -fpatchable-function-entry in that we
emit multi-byte NOPs.

Differential Revision: https://reviews.llvm.org/D72220
---
 llvm/lib/Target/X86/X86MCInstLower.cpp            | 18 +++++--
 llvm/test/CodeGen/X86/patchable-function-entry.ll | 58 +++++++++++++++++++++++
 2 files changed, 73 insertions(+), 3 deletions(-)
 create mode 100644 llvm/test/CodeGen/X86/patchable-function-entry.ll

diff --git a/llvm/lib/Target/X86/X86MCInstLower.cpp b/llvm/lib/Target/X86/X86MCInstLower.cpp
index 3cb9115..e20d1ee 100644
--- a/llvm/lib/Target/X86/X86MCInstLower.cpp
+++ b/llvm/lib/Target/X86/X86MCInstLower.cpp
@@ -1035,9 +1035,11 @@ void X86AsmPrinter::LowerTlsAddr(X86MCInstLower &MCInstLowering,
 /// bytes.  Return the size of nop emitted.
 static unsigned EmitNop(MCStreamer &OS, unsigned NumBytes, bool Is64Bit,
                         const MCSubtargetInfo &STI) {
-  // This works only for 64bit. For 32bit we have to do additional checking if
-  // the CPU supports multi-byte nops.
-  assert(Is64Bit && "EmitNops only supports X86-64");
+  if (!Is64Bit) {
+    // TODO Do additional checking if the CPU supports multi-byte nops.
+    OS.EmitInstruction(MCInstBuilder(X86::NOOP), STI);
+    return 1;
+  }
 
   unsigned NopSize;
   unsigned Opc, BaseReg, ScaleVal, IndexReg, Displacement, SegmentReg;
@@ -1597,6 +1599,16 @@ void X86AsmPrinter::LowerPATCHABLE_FUNCTION_ENTER(const MachineInstr &MI,
 
   NoAutoPaddingScope NoPadScope(*OutStreamer);
 
+  const Function &F = MF->getFunction();
+  if (F.hasFnAttribute("patchable-function-entry")) {
+    unsigned Num;
+    if (F.getFnAttribute("patchable-function-entry")
+            .getValueAsString()
+            .getAsInteger(10, Num))
+      return;
+    EmitNops(*OutStreamer, Num, Subtarget->is64Bit(), getSubtargetInfo());
+    return;
+  }
   // We want to emit the following pattern:
   //
   //   .p2align 1, ...
diff --git a/llvm/test/CodeGen/X86/patchable-function-entry.ll b/llvm/test/CodeGen/X86/patchable-function-entry.ll
new file mode 100644
index 0000000..89d94f8
--- /dev/null
+++ b/llvm/test/CodeGen/X86/patchable-function-entry.ll
@@ -0,0 +1,58 @@
+; RUN: llc -mtriple=i386 %s -o - | FileCheck --check-prefixes=CHECK,NOFSECT,32 %s
+; RUN: llc -mtriple=x86_64 %s -o - | FileCheck --check-prefixes=CHECK,NOFSECT,64 %s
+
+define void @f0() "patchable-function-entry"="0" {
+; CHECK-LABEL: f0:
+; CHECK-NEXT: .Lfunc_begin0:
+; CHECK-NOT:   nop
+; CHECK:       ret
+; CHECK:       .section __patchable_function_entries,"awo",@progbits,f0,unique,0
+; 32:          .p2align 2
+; 32-NEXT:     .long .Lfunc_begin0
+; 64:          .p2align 3
+; 64-NEXT:     .quad .Lfunc_begin0
+  ret void
+}
+
+define void @f1() "patchable-function-entry"="1" {
+; CHECK-LABEL: f1:
+; CHECK:       nop
+; CHECK-NEXT:  ret
+; NOFSECT:     .section __patchable_function_entries,"awo",@progbits,f0,unique,0
+; FSECT:       .section __patchable_function_entries,"awo",@progbits,f1,unique,1
+; 32:          .p2align 2
+; 32-NEXT:     .long .Lfunc_begin1
+; 64:          .p2align 3
+; 64-NEXT:     .quad .Lfunc_begin1
+  ret void
+}
+
+$f3 = comdat any
+define void @f3() "patchable-function-entry"="3" comdat {
+; CHECK-LABEL: f3:
+; 32-COUNT-3:  nop
+; 64:          nopl (%rax)
+; CHECK:       ret
+; NOFSECT:     .section __patchable_function_entries,"aGwo",@progbits,f3,comdat,f3,unique,1
+; FSECT:       .section __patchable_function_entries,"aGwo",@progbits,f3,comdat,f3,unique,2
+; 32:          .p2align 2
+; 32-NEXT:     .long .Lfunc_begin2
+; 64:          .p2align 3
+; 64-NEXT:     .quad .Lfunc_begin2
+  ret void
+}
+
+$f5 = comdat any
+define void @f5() "patchable-function-entry"="5" comdat {
+; CHECK-LABEL: f5:
+; 32-COUNT-5:  nop
+; 64:          nopl 8(%rax,%rax)
+; CHECK-NEXT:  ret
+; NOFSECT      .section __patchable_function_entries,"aGwo",@progbits,f5,comdat,f5,unique,2
+; FSECT:       .section __patchable_function_entries,"aGwo",@progbits,f5,comdat,f5,unique,3
+; 32:          .p2align 2
+; 32-NEXT:     .long .Lfunc_begin3
+; 64:          .p2align 3
+; 64-NEXT:     .quad .Lfunc_begin3
+  ret void
+}
-- 
2.7.4