Re: [PATCH] C inlined assembly for reproducing max<min

[Yonghong Song <yonghong.song@linux.dev>]

On 11/22/23 9:40 AM, Tao Lyu wrote:
> Hi Yonghong,
>
> Thanks for your reply.
> The C inlined assembly code is attached.
> I'm using clang-16, but it still fails.
>
> Best,
> Tao
>
> Signed-off-by: Tao Lyu <tao.lyu@epfl.ch>
> ---
>   .../selftests/bpf/prog_tests/verifier.c       |  2 ++
>   .../selftests/bpf/progs/verifier_range.c      | 25 +++++++++++++++++++
>   2 files changed, 27 insertions(+)
>   create mode 100644 tools/testing/selftests/bpf/progs/verifier_range.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/verifier.c b/tools/testing/selftests/bpf/prog_tests/verifier.c
> index e5c61aa6604a..3a5d746f392d 100644
> --- a/tools/testing/selftests/bpf/prog_tests/verifier.c
> +++ b/tools/testing/selftests/bpf/prog_tests/verifier.c
> @@ -77,6 +77,7 @@
>   #include "verifier_xadd.skel.h"
>   #include "verifier_xdp.skel.h"
>   #include "verifier_xdp_direct_packet_access.skel.h"
> +#include "verifier_range.skel.h"
>   
>   #define MAX_ENTRIES 11
>   
> @@ -184,6 +185,7 @@ void test_verifier_var_off(void)              { RUN(verifier_var_off); }
>   void test_verifier_xadd(void)                 { RUN(verifier_xadd); }
>   void test_verifier_xdp(void)                  { RUN(verifier_xdp); }
>   void test_verifier_xdp_direct_packet_access(void) { RUN(verifier_xdp_direct_packet_access); }
> +void test_verifier_range(void)                { RUN(verifier_range); }
>   
>   static int init_test_val_map(struct bpf_object *obj, char *map_name)
>   {
> diff --git a/tools/testing/selftests/bpf/progs/verifier_range.c b/tools/testing/selftests/bpf/progs/verifier_range.c
> new file mode 100644
> index 000000000000..27597eb8135c
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/verifier_range.c
> @@ -0,0 +1,25 @@
> +#include <linux/bpf.h>
> +#include <bpf/bpf_helpers.h>
> +#include "bpf_misc.h"
> +
> +SEC("?tc")
> +__log_level(2)
> +int test_verifier_range(void)
> +{
> +    asm volatile (
> +        "r5 = 100; \
> +        r5 /= 3; \
> +        w5 >>= 7; \
> +        r5 &= -386969681; \
> +        r5 -= -884670597; \
> +        w0 = w5; \
> +        if w0 & 0x894b6a55 goto +2; \

So actually it is 'if w0 & 0x894b6a55 goto +2' failed
the compilation.

Indeed, the above operation is not supported in llvm.
See
   https://github.com/llvm/llvm-project/blob/main/llvm/lib/Target/BPF/BPFInstrFormats.td#L62-L74
the missing BPFJumpOp<0x4> which corresponds to JSET.

The following llvm patch (on top of llvm-project main branch):

diff --git a/llvm/lib/Target/BPF/BPFInstrFormats.td b/llvm/lib/Target/BPF/BPFInstrFormats.td
index 841d97efc01c..6ed83d877ac0 100644
--- a/llvm/lib/Target/BPF/BPFInstrFormats.td
+++ b/llvm/lib/Target/BPF/BPFInstrFormats.td
@@ -63,6 +63,7 @@ def BPF_JA   : BPFJumpOp<0x0>;
  def BPF_JEQ  : BPFJumpOp<0x1>;
  def BPF_JGT  : BPFJumpOp<0x2>;
  def BPF_JGE  : BPFJumpOp<0x3>;
+def BPF_JSET : BPFJumpOp<0x4>;
  def BPF_JNE  : BPFJumpOp<0x5>;
  def BPF_JSGT : BPFJumpOp<0x6>;
  def BPF_JSGE : BPFJumpOp<0x7>;
diff --git a/llvm/lib/Target/BPF/BPFInstrInfo.td b/llvm/lib/Target/BPF/BPFInstrInfo.td
index 305cbbd34d27..9e75f35efe70 100644
--- a/llvm/lib/Target/BPF/BPFInstrInfo.td
+++ b/llvm/lib/Target/BPF/BPFInstrInfo.td
@@ -246,6 +246,70 @@ class JMP_RI_32<BPFJumpOp Opc, string OpcodeStr, PatLeaf Cond>
    let BPFClass = BPF_JMP32;
  }
  
+class JSET_RR<string OpcodeStr>
+    : TYPE_ALU_JMP<BPF_JSET.Value, BPF_X.Value,
+                   (outs),
+                   (ins GPR:$dst, GPR:$src, brtarget:$BrDst),
+                   "if $dst "#OpcodeStr#" $src goto $BrDst",
+                   []> {
+  bits<4> dst;
+  bits<4> src;
+  bits<16> BrDst;
+
+  let Inst{55-52} = src;
+  let Inst{51-48} = dst;
+  let Inst{47-32} = BrDst;
+  let BPFClass = BPF_JMP;
+}
+
+class JSET_RI<string OpcodeStr>
+    : TYPE_ALU_JMP<BPF_JSET.Value, BPF_K.Value,
+                   (outs),
+                   (ins GPR:$dst, i64imm:$imm, brtarget:$BrDst),
+                   "if $dst "#OpcodeStr#" $imm goto $BrDst",
+                   []> {
+  bits<4> dst;
+  bits<16> BrDst;
+  bits<32> imm;
+
+  let Inst{51-48} = dst;
+  let Inst{47-32} = BrDst;
+  let Inst{31-0} = imm;
+  let BPFClass = BPF_JMP;
+}
+
+class JSET_RR_32<string OpcodeStr>
+    : TYPE_ALU_JMP<BPF_JSET.Value, BPF_X.Value,
+                   (outs),
+                   (ins GPR32:$dst, GPR32:$src, brtarget:$BrDst),
+                   "if $dst "#OpcodeStr#" $src goto $BrDst",
+                   []> {
+  bits<4> dst;
+  bits<4> src;
+  bits<16> BrDst;
+
+  let Inst{55-52} = src;
+  let Inst{51-48} = dst;
+  let Inst{47-32} = BrDst;
+  let BPFClass = BPF_JMP32;
+}
+
+class JSET_RI_32<string OpcodeStr>
+    : TYPE_ALU_JMP<BPF_JSET.Value, BPF_K.Value,
+                   (outs),
+                   (ins GPR32:$dst, i32imm:$imm, brtarget:$BrDst),
+                   "if $dst "#OpcodeStr#" $imm goto $BrDst",
+                   []> {
+  bits<4> dst;
+  bits<16> BrDst;
+  bits<32> imm;
+
+  let Inst{51-48} = dst;
+  let Inst{47-32} = BrDst;
+  let Inst{31-0} = imm;
+  let BPFClass = BPF_JMP32;
+}
+
  multiclass J<BPFJumpOp Opc, string OpcodeStr, PatLeaf Cond, PatLeaf Cond32> {
    def _rr : JMP_RR<Opc, OpcodeStr, Cond>;
    def _ri : JMP_RI<Opc, OpcodeStr, Cond>;
@@ -265,6 +329,10 @@ defm JULT : J<BPF_JLT, "<", BPF_CC_LTU, BPF_CC_LTU_32>;
  defm JULE : J<BPF_JLE, "<=", BPF_CC_LEU, BPF_CC_LEU_32>;
  defm JSLT : J<BPF_JSLT, "s<", BPF_CC_LT, BPF_CC_LT_32>;
  defm JSLE : J<BPF_JSLE, "s<=", BPF_CC_LE, BPF_CC_LE_32>;
+def JSET_RR    : JSET_RR<"&">;
+def JSET_RI    : JSET_RI<"&">;
+def JSET_RR_32 : JSET_RR_32<"&">;
+def JSET_RI_32 : JSET_RI_32<"&">;
  }
  
  // ALU instructions

can solve your inline asm issue. We will discuss whether llvm compiler
should be implementing this instruction from source or not.


> +        r2 = 1; \
> +        r2 = 1; \
> +        r0 = 0; \
> +        "
> +    );
> +    return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";