bpf: avoid load from read-only sections

If users tried to have a structure decl/init code like below
   struct test_t t = { .memeber1 = 45 };
It is very likely that compiler will generate a readonly section
to hold up the init values for variable t. Later load of t members,
e.g., t.member1 will result in a read from readonly section.

BPF program cannot handle relocation. This will force users to
write:
  struct test_t t = {};
  t.member1 = 45;
This is just inconvenient and unintuitive.

This patch addresses this issue by implementing BPF PreprocessISelDAG.
For any load from a global constant structure or an global array of
constant struct, it attempts to
translate it into a constant directly. The traversal of the
constant struct and other constant data structures are similar
to where the assembler emits read-only sections.

Four different unit test cases are also added to cover
different scenarios.

Signed-off-by: Yonghong Song <yhs@fb.com>

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@305560 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 41 insertions, 0 deletions

diff --git a/test/CodeGen/BPF/rodata_3.ll b/test/CodeGen/BPF/rodata_3.ll
new file mode 100644
index 00000000000..814ce764546
--- /dev/null
+++ b/test/CodeGen/BPF/rodata_3.ll
@@ -0,0 +1,41 @@
+; REQUIRES: x86_64-linux
+; RUN: llc < %s -march=bpfel -verify-machineinstrs | FileCheck --check-prefix=CHECK-EL %s
+; RUN: llc < %s -march=bpfeb -verify-machineinstrs | FileCheck --check-prefix=CHECK-EB %s
+;
+; This test requires little-endian host, so we specific x86_64-linux here.
+; Source code:
+; struct test_t1 {
+;   char a;
+;   int b, c, d;
+; };
+;
+; struct test_t1 g;
+; int test()
+; {
+;   struct test_t1 t1 = {.a = 1};
+;   g = t1;
+;   return 0;
+; }
+
+%struct.test_t1 = type { i8, i32, i32, i32 }
+
+@test.t1 = private unnamed_addr constant %struct.test_t1 { i8 1, i32 0, i32 0, i32 0 }, align 4
+@g = common local_unnamed_addr global %struct.test_t1 zeroinitializer, align 4
+
+; Function Attrs: nounwind
+define i32 @test() local_unnamed_addr #0 {
+entry:
+    tail call void @llvm.memcpy.p0i8.p0i8.i64(i8* getelementptr inbounds (%struct.test_t1, %struct.test_t1* @g, i64 0, i32 0), i8* getelementptr inbounds (%struct.test_t1, %struct.test_t1* @test.t1, i64 0, i32 0), i64 16, i32 4, i1 false)
+; CHECK-EL:  r2 = 1
+; CHECK-EL:  *(u32 *)(r1 + 0) = r2
+; CHECK-EB:  r2 = 16777216
+; CHECK-EB:  *(u32 *)(r1 + 0) = r2
+    ret i32 0
+}
+; CHECK-EL:  .section .rodata.cst16,"aM",@progbits,16
+; CHECK-EB:  .section .rodata.cst16,"aM",@progbits,16
+
+declare void @llvm.memcpy.p0i8.p0i8.i64(i8* nocapture writeonly, i8* nocapture readonly, i64, i32, i1) #1
+
+attributes #0 = { nounwind }
+attributes #1 = { argmemonly nounwind }