[BPF] do compile-once run-everywhere relocation for bitfields

A bpf specific clang intrinsic is introduced:
   u32 __builtin_preserve_field_info(member_access, info_kind)
Depending on info_kind, different information will
be returned to the program. A relocation is also
recorded for this builtin so that bpf loader can
patch the instruction on the target host.
This clang intrinsic is used to get certain information
to facilitate struct/union member relocations.

The offset relocation is extended by 4 bytes to
include relocation kind.
Currently supported relocation kinds are
 enum {
    FIELD_BYTE_OFFSET = 0,
    FIELD_BYTE_SIZE,
    FIELD_EXISTENCE,
    FIELD_SIGNEDNESS,
    FIELD_LSHIFT_U64,
    FIELD_RSHIFT_U64,
 };
for __builtin_preserve_field_info. The old
access offset relocation is covered by
    FIELD_BYTE_OFFSET = 0.

An example:
struct s {
    int a;
    int b1:9;
    int b2:4;
};
enum {
    FIELD_BYTE_OFFSET = 0,
    FIELD_BYTE_SIZE,
    FIELD_EXISTENCE,
    FIELD_SIGNEDNESS,
    FIELD_LSHIFT_U64,
    FIELD_RSHIFT_U64,
};

void bpf_probe_read(void *, unsigned, const void *);
int field_read(struct s *arg) {
  unsigned long long ull = 0;
  unsigned offset = __builtin_preserve_field_info(arg->b2, FIELD_BYTE_OFFSET);
  unsigned size = __builtin_preserve_field_info(arg->b2, FIELD_BYTE_SIZE);
 #ifdef USE_PROBE_READ
  bpf_probe_read(&ull, size, (const void *)arg + offset);
  unsigned lshift = __builtin_preserve_field_info(arg->b2, FIELD_LSHIFT_U64);
 #if __BYTE_ORDER__ == __ORDER_BIG_ENDIAN__
  lshift = lshift + (size << 3) - 64;
 #endif
 #else
  switch(size) {
  case 1:
    ull = *(unsigned char *)((void *)arg + offset); break;
  case 2:
    ull = *(unsigned short *)((void *)arg + offset); break;
  case 4:
    ull = *(unsigned int *)((void *)arg + offset); break;
  case 8:
    ull = *(unsigned long long *)((void *)arg + offset); break;
  }
  unsigned lshift = __builtin_preserve_field_info(arg->b2, FIELD_LSHIFT_U64);
 #endif
  ull <<= lshift;
  if (__builtin_preserve_field_info(arg->b2, FIELD_SIGNEDNESS))
    return (long long)ull >> __builtin_preserve_field_info(arg->b2, FIELD_RSHIFT_U64);
  return ull >> __builtin_preserve_field_info(arg->b2, FIELD_RSHIFT_U64);
}

There is a minor overhead for bpf_probe_read() on big endian.

The code and relocation generated for field_read where bpf_probe_read() is
used to access argument data on little endian mode:
        r3 = r1
        r1 = 0
        r1 = 4  <=== relocation (FIELD_BYTE_OFFSET)
        r3 += r1
        r1 = r10
        r1 += -8
        r2 = 4  <=== relocation (FIELD_BYTE_SIZE)
        call bpf_probe_read
        r2 = 51 <=== relocation (FIELD_LSHIFT_U64)
        r1 = *(u64 *)(r10 - 8)
        r1 <<= r2
        r2 = 60 <=== relocation (FIELD_RSHIFT_U64)
        r0 = r1
        r0 >>= r2
        r3 = 1  <=== relocation (FIELD_SIGNEDNESS)
        if r3 == 0 goto LBB0_2
        r1 s>>= r2
        r0 = r1
LBB0_2:
        exit

Compare to the above code between relocations FIELD_LSHIFT_U64 and
FIELD_LSHIFT_U64, the code with big endian mode has four more
instructions.
        r1 = 41   <=== relocation (FIELD_LSHIFT_U64)
        r6 += r1
        r6 += -64
        r6 <<= 32
        r6 >>= 32
        r1 = *(u64 *)(r10 - 8)
        r1 <<= r6
        r2 = 60   <=== relocation (FIELD_RSHIFT_U64)

The code and relocation generated when using direct load.
        r2 = 0
        r3 = 4
        r4 = 4
        if r4 s> 3 goto LBB0_3
        if r4 == 1 goto LBB0_5
        if r4 == 2 goto LBB0_6
        goto LBB0_9
LBB0_6:                                 # %sw.bb1
        r1 += r3
        r2 = *(u16 *)(r1 + 0)
        goto LBB0_9
LBB0_3:                                 # %entry
        if r4 == 4 goto LBB0_7
        if r4 == 8 goto LBB0_8
        goto LBB0_9
LBB0_8:                                 # %sw.bb9
        r1 += r3
        r2 = *(u64 *)(r1 + 0)
        goto LBB0_9
LBB0_5:                                 # %sw.bb
        r1 += r3
        r2 = *(u8 *)(r1 + 0)
        goto LBB0_9
LBB0_7:                                 # %sw.bb5
        r1 += r3
        r2 = *(u32 *)(r1 + 0)
LBB0_9:                                 # %sw.epilog
        r1 = 51
        r2 <<= r1
        r1 = 60
        r0 = r2
        r0 >>= r1
        r3 = 1
        if r3 == 0 goto LBB0_11
        r2 s>>= r1
        r0 = r2
LBB0_11:                                # %sw.epilog
        exit

Considering verifier is able to do limited constant
propogation following branches. The following is the
code actually traversed.
        r2 = 0
        r3 = 4   <=== relocation
        r4 = 4   <=== relocation
        if r4 s> 3 goto LBB0_3
LBB0_3:                                 # %entry
        if r4 == 4 goto LBB0_7
LBB0_7:                                 # %sw.bb5
        r1 += r3
        r2 = *(u32 *)(r1 + 0)
LBB0_9:                                 # %sw.epilog
        r1 = 51   <=== relocation
        r2 <<= r1
        r1 = 60   <=== relocation
        r0 = r2
        r0 >>= r1
        r3 = 1
        if r3 == 0 goto LBB0_11
        r2 s>>= r1
        r0 = r2
LBB0_11:                                # %sw.epilog
        exit

For native load case, the load size is calculated to be the
same as the size of load width LLVM otherwise used to load
the value which is then used to extract the bitfield value.

Differential Revision: https://reviews.llvm.org/D67980

git-svn-id: https://llvm.org/svn/llvm-project/cfe/trunk@374099 91177308-0d34-0410-b5e6-96231b3b80d8

5 files changed, 41 insertions, 0 deletions

diff --git a/include/clang/Basic/BuiltinsBPF.def b/include/clang/Basic/BuiltinsBPF.def
new file mode 100644
index 0000000000..bd96b9ef53
--- /dev/null
+++ b/include/clang/Basic/BuiltinsBPF.def
@@ -0,0 +1,24 @@
+//===--- BuiltinsBPF.def - BPF Builtin function database --------*- C++ -*-===//
+//
+// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
+// See https://llvm.org/LICENSE.txt for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+//
+//===----------------------------------------------------------------------===//
+//
+// This file defines the BPF-specific builtin function database.  Users of
+// this file must define the BUILTIN macro to make use of this information.
+//
+//===----------------------------------------------------------------------===//
+
+// The format of this database matches clang/Basic/Builtins.def.
+
+#if defined(BUILTIN) && !defined(TARGET_BUILTIN)
+#   define TARGET_BUILTIN(ID, TYPE, ATTRS, FEATURE) BUILTIN(ID, TYPE, ATTRS)
+#endif
+
+// Get record field information.
+TARGET_BUILTIN(__builtin_preserve_field_info, "Ui.", "t", "")
+
+#undef BUILTIN
+#undef TARGET_BUILTIN
diff --git a/include/clang/Basic/DiagnosticSemaKinds.td b/include/clang/Basic/DiagnosticSemaKinds.td
index 23cedcab27..d0d1a7d4ea 100644
--- a/include/clang/Basic/DiagnosticSemaKinds.td
+++ b/include/clang/Basic/DiagnosticSemaKinds.td
@@ -9954,6 +9954,11 @@ def err_builtin_launder_invalid_arg : Error<
   "%select{non-pointer|function pointer|void pointer}0 argument to "
   "'__builtin_launder' is not allowed">;
 
+def err_preserve_field_info_not_field : Error<
+  "__builtin_preserve_field_info argument %0 not a field access">;
+def err_preserve_field_info_not_const: Error<
+  "__builtin_preserve_field_info argument %0 not a constant">;
+
 def err_bit_cast_non_trivially_copyable : Error<
   "__builtin_bit_cast %select{source|destination}0 type must be trivially copyable">;
 def err_bit_cast_type_size_mismatch : Error<
diff --git a/include/clang/Basic/TargetBuiltins.h b/include/clang/Basic/TargetBuiltins.h
index 50262fa310..0e2f0753b0 100644
--- a/include/clang/Basic/TargetBuiltins.h
+++ b/include/clang/Basic/TargetBuiltins.h
@@ -52,6 +52,16 @@ namespace clang {
   };
   }
 
+  /// BPF builtins
+  namespace BPF {
+  enum {
+    LastTIBuiltin = clang::Builtin::FirstTSBuiltin - 1,
+  #define BUILTIN(ID, TYPE, ATTRS) BI##ID,
+  #include "clang/Basic/BuiltinsBPF.def"
+    LastTSBuiltin
+  };
+  }
+
   /// PPC builtins
   namespace PPC {
     enum {
diff --git a/include/clang/Sema/Sema.h b/include/clang/Sema/Sema.h
index 28180ed6d1..bb05a08e54 100644
--- a/include/clang/Sema/Sema.h
+++ b/include/clang/Sema/Sema.h
@@ -11056,6 +11056,7 @@ private:
   bool CheckARMBuiltinFunctionCall(unsigned BuiltinID, CallExpr *TheCall);
 
   bool CheckAArch64BuiltinFunctionCall(unsigned BuiltinID, CallExpr *TheCall);
+  bool CheckBPFBuiltinFunctionCall(unsigned BuiltinID, CallExpr *TheCall);
   bool CheckHexagonBuiltinFunctionCall(unsigned BuiltinID, CallExpr *TheCall);
   bool CheckHexagonBuiltinCpu(unsigned BuiltinID, CallExpr *TheCall);
   bool CheckHexagonBuiltinArgument(unsigned BuiltinID, CallExpr *TheCall);
diff --git a/include/clang/module.modulemap b/include/clang/module.modulemap
index f7bf482fbb..0f9b7bb908 100644
--- a/include/clang/module.modulemap
+++ b/include/clang/module.modulemap
@@ -34,6 +34,7 @@ module Clang_Basic {
   textual header "Basic/BuiltinsAArch64.def"
   textual header "Basic/BuiltinsAMDGPU.def"
   textual header "Basic/BuiltinsARM.def"
+  textual header "Basic/BuiltinsBPF.def"
   textual header "Basic/Builtins.def"
   textual header "Basic/BuiltinsHexagon.def"
   textual header "Basic/BuiltinsLe64.def"