On 8/15/23 7:19 AM, Jose E. Marchesi wrote:
Hello.
The selftest progs/verifier_masking.c contains inline assembly code
like:
w1 = 0xffffffff;
The 32-bit immediate of that instruction is signed. Therefore, GAS
complains that the above instruction overflows its field:
/tmp/ccNOXFQy.s:46: Error: signed immediate out of range, shall fit in 32 bits
The llvm assembler is likely relying on signed overflow for the above to
work.
Not really.
def _ri_32 : ALU_RI<BPF_ALU, Opc, off,
(outs GPR32:$dst),
(ins GPR32:$src2, i32imm:$imm),
"$dst "#OpcodeStr#" $imm",
[(set GPR32:$dst, (OpNode GPR32:$src2,
i32immSExt32:$imm))]>;
If generating from source, the pattern
[(set GPR32:$dst, (OpNode GPR32:$src2, i32immSExt32:$imm))]
so value 0xffffffff is not SExt32 and it won't match and
eventually a LDimm_64 insn will be generated.
But for inline asm, we will have
(outs GPR32:$dst)
(ins GPR32:$src2, i32imm:$imm)
and i32imm is defined as
def i32imm : Operand<i32>;
which is a unsigned 32bit value, so it is recognized properly
and the insn is encoded properly.
Using negative numbers to denote masks is ugly and obfuscating (for
non-obvious cases like -1/0xffffffff) so I suggest we introduce a
pseudo-op so we can do:
w1 = %mask(0xffffffff)
I changed above
w1 = 0xffffffff;
to
w1 = %mask(0xffffffff)
and hit the following compilation failure.
progs/verifier_masking.c:54:9: error: invalid % escape in inline
assembly string
53 | asm volatile (" \
| ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
54 | w1 = %mask(0xffffffff); \
| ^
1 error generated.
Do you have documentation what is '%mask' thing?
allowing the assembler to do the right thing (TM) converting and
checking that the mask is valid and not relying on UB.
Thoughts?
