On 12/8/23 8:25 PM, Alexei Starovoitov wrote:
On Fri, Dec 8, 2023 at 8:15 PM Yonghong Song <yonghong.song@xxxxxxxxx> wrote:
On 12/8/23 8:05 PM, Alexei Starovoitov wrote:
On Fri, Dec 8, 2023 at 2:04 PM Andrii Nakryiko
<andrii.nakryiko@xxxxxxxxx> wrote:
I feel like embedding some sort of ID inside the instruction is very..
unusual, shall we say?
yeah. no magic numbers inside insns pls.
I don't like JA_CFG name, since I read CFG as control flow graph,
while you probably meant CFG as configurable.
How about BPF_JA_OR_NOP ?
Then in combination with BPF_JMP or BPF_JMP32 modifier
the insn->off|imm will be used.
1st bit in src_reg can indicate the default action: nop or jmp.
In asm it may look like asm("goto_or_nop +5")
How does the C source code looks like in order to generate
BPF_JA_OR_NOP insn? Any source examples?
It will be in inline asm only. The address of that insn will
be taken either via && or via asm (".long %l[label]").
From llvm pov both should go through the same relo creation logic. I hope :)
A hack in llvm below with an example, could you check whether the C
syntax and object dump result
is what you want to see?
diff --git a/llvm/lib/Target/BPF/AsmParser/BPFAsmParser.cpp
b/llvm/lib/Target/BPF/AsmParser/BPFAsmParser.cpp
index 90697c6645be..38b1cbc31f9a 100644
--- a/llvm/lib/Target/BPF/AsmParser/BPFAsmParser.cpp
+++ b/llvm/lib/Target/BPF/AsmParser/BPFAsmParser.cpp
@@ -231,6 +231,7 @@ public:
.Case("call", true)
.Case("goto", true)
.Case("gotol", true)
+ .Case("goto_or_nop", true)
.Case("*", true)
.Case("exit", true)
.Case("lock", true)
@@ -259,6 +260,7 @@ public:
.Case("bswap64", true)
.Case("goto", true)
.Case("gotol", true)
+ .Case("goto_or_nop", true)
.Case("ll", true)
.Case("skb", true)
.Case("s", true)
diff --git a/llvm/lib/Target/BPF/BPFInstrInfo.td
b/llvm/lib/Target/BPF/BPFInstrInfo.td
index 5972c9d49c51..a953d10429bf 100644
--- a/llvm/lib/Target/BPF/BPFInstrInfo.td
+++ b/llvm/lib/Target/BPF/BPFInstrInfo.td
@@ -592,6 +592,19 @@ class BRANCH<BPFJumpOp Opc, string OpcodeStr,
list<dag> Pattern>
let BPFClass = BPF_JMP;
}
+class BRANCH_OR_NOP<BPFJumpOp Opc, string OpcodeStr, list<dag> Pattern>
+ : TYPE_ALU_JMP<Opc.Value, BPF_K.Value,
+ (outs),
+ (ins brtarget:$BrDst),
+ !strconcat(OpcodeStr, " $BrDst"),
+ Pattern> {
+ bits<16> BrDst;
+
+ let Inst{47-32} = BrDst;
+ let Inst{31-0} = 1;
+ let BPFClass = BPF_JMP;
+}
+
class BRANCH_LONG<BPFJumpOp Opc, string OpcodeStr, list<dag> Pattern>
: TYPE_ALU_JMP<Opc.Value, BPF_K.Value,
(outs),
@@ -632,6 +645,7 @@ class CALLX<string OpcodeStr>
let isBranch = 1, isTerminator = 1, hasDelaySlot=0, isBarrier = 1 in {
def JMP : BRANCH<BPF_JA, "goto", [(br bb:$BrDst)]>;
def JMPL : BRANCH_LONG<BPF_JA, "gotol", []>;
+ def JMP_OR_NOP : BRANCH_OR_NOP<BPF_JA, "goto_or_nop", []>;
}
// Jump and link
And an example,
[ ~/tmp1/gotol]$ cat t.c
int bar(void);
int foo()
{
int a, b;
asm volatile goto ("r0 = 0; \
goto_or_nop %l[label]; \
r2 = 2; \
r3 = 3; \
"::::label);
a = bar();
label:
b = 20 * a;
return b;
}
[ ~/tmp1/gotol]$ clang --target=bpf -O2 -S t.c
[ ~/tmp1/gotol]$ cat t.s
.text
.file "t.c"
.globl foo # -- Begin function foo
.p2align 3
.type foo,@function
foo: # @foo
# %bb.0: # %entry
r0 = 0
#APP
r0 = 0
goto_or_nop LBB0_2
r2 = 2
r3 = 3
#NO_APP
# %bb.1: # %asm.fallthrough
call bar
r0 *= 20
LBB0_2: # Block address taken
# %label
# Label of block must be emitted
exit
.Lfunc_end0:
.size foo, .Lfunc_end0-foo
# -- End function
.addrsig
[ ~/tmp1/gotol]$ clang --target=bpf -O2 -c t.c
[ ~/tmp1/gotol]$ llvm-objdump -dr t.o
t.o: file format elf64-bpf
Disassembly of section .text:
0000000000000000 <foo>:
0: b7 00 00 00 00 00 00 00 r0 = 0x0
1: b7 00 00 00 00 00 00 00 r0 = 0x0
2: 05 00 04 00 01 00 00 00 goto_or_nop +0x4 <LBB0_2>
3: b7 02 00 00 02 00 00 00 r2 = 0x2
4: b7 03 00 00 03 00 00 00 r3 = 0x3
5: 85 10 00 00 ff ff ff ff call -0x1
0000000000000028: R_BPF_64_32 bar
6: 27 00 00 00 14 00 00 00 r0 *= 0x14
0000000000000038 <LBB0_2>:
7: 95 00 00 00 00 00 00 00 exit
[ ~/tmp1/gotol]$
