[PATCH] D61810: [BPF] add a new intrinsic preserve_di_access_index
Yonghong Song via Phabricator via llvm-commits
llvm-commits at lists.llvm.org
Fri May 10 15:39:31 PDT 2019
yonghong-song created this revision.
yonghong-song added reviewers: eli.friedman, ast.
Herald added subscribers: llvm-commits, arphaman, kosarev.
Herald added a project: LLVM.
For background of BPF CO-RE project, please refer to
http://vger.kernel.org/bpfconf2019.html
In summary, BPF CO-RE intends to compile bpf programs
adjustable on struct/union layout change so the same
program can run on multiple kernels with adjustment
before loading based on native kernel structures.
In order to do this, we need keep track of GEP(getelementptr)
instruction base and result debuginfo types, so we
can adjust on the host based on kernel BTF info.
Capturing such information as an IR optimization is hard
as various optimization may have tweaked GEP and also
union is replaced by structure it is impossible to track
fieldindex for union member accesses.
An intrinsic function, preserve_di_access_index, is introducted.
naddr = preserve_di_access_index(addr, base, type_name, access_index)
here,
addr: the previous getelementptr result, used as the result value do
program semantics are kept intact.
base: the base of previous getelementptr, used for later
code generation with new relocatable access offset.
type_name: the struct/union type name if available, can be used to
match corresponding types in debuginfo.
access_index: the access index based on user/debuginfo types.
naddr: the result, having the same type as "addr".
For example, for the following example,
$ cat test.c
struct sk_buff {
int i;
int b1:1;
int b2:2;
union {
struct {
int o1;
int o2;
} o;
struct {
char flags;
char dev_id;
} dev;
int netid;
} u[10];
};
static int (*bpf_probe_read)(void *dst, int size, void *unsafe_ptr)
= (void *) 4;
int bpf_prog(struct sk_buff *ctx) {
char dev_id;
bpf_probe_read(&dev_id, sizeof(char), &ctx->u[5].dev.dev_id);
return dev_id;
}
$ clang -target bpf -O2 -g -emit-llvm -S -mllvm -print-before-all test.c >& log
The generated IR looks like below:
...
define dso_local i32 @bpf_prog(%struct.sk_buff*) #0 !dbg !15 {
%2 = alloca %struct.sk_buff*, align 8
%3 = alloca i8, align 1
store %struct.sk_buff* %0, %struct.sk_buff** %2, align 8, !tbaa !45
call void @llvm.dbg.declare(metadata %struct.sk_buff** %2, metadata !43, metadata !DIExpression()), !dbg !49
call void @llvm.lifetime.start.p0i8(i64 1, i8* %3) #4, !dbg !50
call void @llvm.dbg.declare(metadata i8* %3, metadata !44, metadata !DIExpression()), !dbg !51
%4 = load i32 (i8*, i32, i8*)*, i32 (i8*, i32, i8*)** @bpf_probe_read, align 8, !dbg !52, !tbaa !45
%5 = load %struct.sk_buff*, %struct.sk_buff** %2, align 8, !dbg !53, !tbaa !45
%6 = getelementptr inbounds %struct.sk_buff, %struct.sk_buff* %5, i32 0, i32 2, !dbg !54
%7 = call [10 x %union.anon]*
@llvm.preserve.di.access.index.p0a10s_union.anons.p0a10s_union.anons.p0s_struct.sk_buffs(
[10 x %union.anon]* %6, %struct.sk_buff* %5,
i8* getelementptr inbounds ([8 x i8], [8 x i8]* @0, i32 0, i32 0), i32 3), !dbg !54
%8 = getelementptr inbounds [10 x %union.anon], [10 x %union.anon]* %7, i64 0, i64 5, !dbg !53
%9 = call %union.anon* @llvm.preserve.di.access.index.p0s_union.anons.p0s_union.anons.p0a10s_union.anons(
%union.anon* %8, [10 x %union.anon]* %7,
i8* getelementptr inbounds ([1 x i8], [1 x i8]* @1, i32 0, i32 0), i32 5), !dbg !53
%10 = call %union.anon* @llvm.preserve.di.access.index.p0s_union.anons.p0s_union.anons.p0s_union.anons(
%union.anon* %9, %union.anon* %9, i8* getelementptr inbounds ([1 x i8], [1 x i8]* @2, i32 0, i32 0), i32 1), !dbg !55
%11 = bitcast %union.anon* %10 to %struct.anon.0*, !dbg !55
%12 = getelementptr inbounds %struct.anon.0, %struct.anon.0* %11, i32 0, i32 1, !dbg !56
%13 = call i8* @llvm.preserve.di.access.index.p0i8.p0i8.p0s_struct.anon.0s(i8* %12, %struct.anon.0* %11,
i8* getelementptr inbounds ([1 x i8], [1 x i8]* @3, i32 0, i32 0), i32 1), !dbg !56
%14 = call i32 %4(i8* %3, i32 1, i8* %13), !dbg !52
%15 = load i8, i8* %3, align 1, !dbg !57, !tbaa !58
%16 = sext i8 %15 to i32, !dbg !57
call void @llvm.lifetime.end.p0i8(i64 1, i8* %3) #4, !dbg !59
ret i32 %16, !dbg !60
}
For &ctx->u[5].dev.dev_id,
. The first getelementptr (%6 = ...) has index 2 based on IR layout, and subsequent
preserve_di_access_index (%7 = ...) has index 3 which reflects the debuginfo type layout.
. The second getelementptr (%8 = ...) has index 5 which is the same as preserve_di_access_index
(%9 = ...) for array subscript.
. The instruction "%10 ..." is a call to preserve_di_access_index, which encodes the union member
access index "1". Such information is lost in the original IR.
. The third getelementptr (%12 = ...) has index 1 anonymous struct member "dev_id". The
subsequent preserve_di_access_index also has the index "1".
Basically, traversing the use-def chain recursively for the 3rd argument of bpf_probe_read() and
examining all preserve_di_access_index calls, the original struct/union/array access index
can be achieved.
The original type name is also kept by preserve_di_access_index call.
For example, for "%7 = ...", the second argument is a constantexpr getelementptr instrunction
with base
@0 = private unnamed_addr constant [8 x i8] c"sk_buff\00", align 1
which indicates the type name sk_buff. The type name can be used to search in DebugInfo types
to find the precise definition.
Finally, the intrinsic
naddr = preserve_di_access_index(addr, base, type_name, access_index)
can be simply transformed to
naddr = addr
or deleted by replacing all uses of "naddr" with "addr".
Repository:
rL LLVM
https://reviews.llvm.org/D61810
Files:
include/llvm/IR/IRBuilder.h
include/llvm/IR/Intrinsics.td
Index: include/llvm/IR/Intrinsics.td
===================================================================
--- include/llvm/IR/Intrinsics.td
+++ include/llvm/IR/Intrinsics.td
@@ -1021,6 +1021,11 @@
// Intrinsic to detect whether its argument is a constant.
def int_is_constant : Intrinsic<[llvm_i1_ty], [llvm_any_ty], [IntrNoMem], "llvm.is.constant">;
+// Intrinsic to preserve the original fieldindex.
+def int_preserve_di_access_index : Intrinsic<[llvm_anyptr_ty],
+ [llvm_anyptr_ty, llvm_anyptr_ty,
+ llvm_ptr_ty, llvm_i32_ty],
+ [IntrNoMem, ImmArg<3>]>;
//===-------------------------- Masked Intrinsics -------------------------===//
//
Index: include/llvm/IR/IRBuilder.h
===================================================================
--- include/llvm/IR/IRBuilder.h
+++ include/llvm/IR/IRBuilder.h
@@ -2267,6 +2267,28 @@
return V;
}
+ Value *CreatePreserveDIAccessIndex(Value *Ptr, Value *Base,
+ StringRef BaseTypeName,
+ unsigned AccessIndex) {
+ assert(isa<PointerType>(Ptr->getType()) &&
+ "Invalid Ptr type for preserve.di.access.index.");
+ auto *PtrType = Ptr->getType();
+
+ assert(isa<PointerType>(Base->getType()) &&
+ "Invalid Ptr type for preserve.di.access.index.");
+ auto *BaseType = Base->getType();
+
+ Module *M = BB->getParent()->getParent();
+ Function *FnPreserveDIAccessIndex = Intrinsic::getDeclaration(
+ M, Intrinsic::preserve_di_access_index, {PtrType, PtrType, BaseType});
+ Value *RecordName = CreateGlobalStringPtr(BaseTypeName);
+ Value *IndexValue = getInt32(AccessIndex);
+ CallInst *Fn = CreateCall(FnPreserveDIAccessIndex,
+ {Ptr, Base, RecordName, IndexValue});
+
+ return Fn;
+ }
+
private:
/// Helper function that creates an assume intrinsic call that
/// represents an alignment assumption on the provided Ptr, Mask, Type
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