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BPF: move AbstractMemberAccess and PreserveDIType passes to EP_EarlyAsPossible

Authored by yonghong-song on Sep 2 2020, 10:56 PM.

Description

BPF: move AbstractMemberAccess and PreserveDIType passes to EP_EarlyAsPossible

Move abstractMemberAccess and PreserveDIType passes as early as
possible, right after clang code generation.

Currently, compiler may transform the above code

p1 = llvm.bpf.builtin.preserve.struct.access(base, 0, 0);
p2 = llvm.bpf.builtin.preserve.struct.access(p1, 1, 2);
a = llvm.bpf.builtin.preserve_field_info(p2, EXIST);
if (a) {
  p1 = llvm.bpf.builtin.preserve.struct.access(base, 0, 0);
  p2 = llvm.bpf.builtin.preserve.struct.access(p1, 1, 2);
  bpf_probe_read(buf, buf_size, p2);
}

to

p1 = llvm.bpf.builtin.preserve.struct.access(base, 0, 0);
p2 = llvm.bpf.builtin.preserve.struct.access(p1, 1, 2);
a = llvm.bpf.builtin.preserve_field_info(p2, EXIST);
if (a) {
  bpf_probe_read(buf, buf_size, p2);
}

and eventually assembly code looks like

reloc_exist = 1;
reloc_member_offset = 10; //calculate member offset from base
p2 = base + reloc_member_offset;
if (reloc_exist) {
  bpf_probe_read(bpf, buf_size, p2);
}

if during libbpf relocation resolution, reloc_exist is actually
resolved to 0 (not exist), reloc_member_offset relocation cannot
be resolved and will be patched with illegal instruction.
This will cause verifier failure.

This patch attempts to address this issue by do chaining
analysis and replace chains with special globals right
after clang code gen. This will remove the cse possibility
described in the above. The IR typically looks like

%6 = load @llvm.sk_buff:0:50$0:0:0:2:0
%7 = bitcast %struct.sk_buff* %2 to i8*
%8 = getelementptr i8, i8* %7, %6

for a particular address computation relocation.

But this transformation has another consequence, code sinking
may happen like below:

PHI = <possibly different @preserve_*_access_globals>
%7 = bitcast %struct.sk_buff* %2 to i8*
%8 = getelementptr i8, i8* %7, %6

For such cases, we will not able to generate relocations since
multiple relocations are merged into one.

This patch introduced a passthrough builtin
to prevent such optimization. Looks like inline assembly has more
impact for optimizaiton, e.g., inlining. Using passthrough has
less impact on optimizations.

A new IR pass is introduced at the beginning of target-dependent
IR optimization, which does:

  • report fatal error if any reloc global in PHI nodes
  • remove all bpf passthrough builtin functions

Changes for existing CORE tests:

  • for clang tests, add "-Xclang -disable-llvm-passes" flags to avoid builtin->reloc_global transformation so the test is still able to check correctness for clang generated IR.
  • for llvm CodeGen/BPF tests, add "opt -O2 <ir_file> | llvm-dis" command before "llc" command since "opt" is needed to call newly-placed builtin->reloc_global transformation. Add target triple in the IR file since "opt" requires it.
  • Since target triple is added in IR file, if a test may produce different results for different endianness, two tests will be created, one for bpfeb and another for bpfel, e.g., some tests for relocation of lshift/rshift of bitfields.
  • field-reloc-bitfield-1.ll has different relocations compared to old codes. This is because for the structure in the test, new code returns struct layout alignment 4 while old code is 8. Align 8 is more precise and permits double load. With align 4, the new mechanism uses 4-byte load, so generating different relocations.
  • test intrinsic-transforms.ll is removed. This is used to test cse on intrinsics so we do not lose metadata. Now metadata is attached to global and not instruction, it won't get lost with cse.

Differential Revision: https://reviews.llvm.org/D87153