On Mon, 13 Mar 2023 23:45:33 +0100
Nina Schoetterl-Glausch <nsg@xxxxxxxxxxxxx> wrote:
[...]
> > > +/*
> > > + * BRANCH AND SAVE, register register variant.
> > > + * Saves the next instruction address (address from PSW + length of instruction)
> > > + * to the first register. No branch is taken in this test, because 0 is
> > > + * specified as target.
> > > + * BASR does *not* perform a relative address calculation with an intermediate.
> > > + */
> > > +static void test_basr(void)
> > > +{
> > > + uint64_t ret_addr, after_ex;
> > > +
> > > + report_prefix_push("BASR");
> > > + asm volatile ( ".pushsection .rodata\n"
> >
> > you use .text.ex_bras in the next test, why not something like that here
> > (and everywhere else) too?
>
> In the test below we branch to the code in .text.ex_bras.
> In all other tests the instruction in .rodata is just an operand of the execute instruction,
> and it doesn't get modified.
> As for the bras test having a suffix, I guess it's pretty arbitrary, but since it's a handful
> of instructions instead of just one, it felt substantial enough to warrant one.
>
we discussed this offline :)
> >
> > > + "0: basr %[ret_addr],0\n"
> > > + " .popsection\n"
> > > +
> > > + " larl %[after_ex],1f\n"
> > > + " exrl 0,0b\n"
> > > + "1:\n"
> > > + : [ret_addr] "=d" (ret_addr),
> > > + [after_ex] "=d" (after_ex)
> > > + );
> > > +
> > > + report(ret_addr == after_ex, "return address after EX");
> > > + report_prefix_pop();
> > > +}
> > > +
> > > +/*
> > > + * BRANCH RELATIVE AND SAVE.
> > > + * According to PoP (Branch-Address Generation), the address calculated relative
> > > + * to the instruction address is relative to BRAS when it is the target of an
> > > + * execute-type instruction, not relative to the execute-type instruction.
> > > + */
> > > +static void test_bras(void)
> > > +{
> > > + uint64_t after_target, ret_addr, after_ex, branch_addr;
> > > +
> > > + report_prefix_push("BRAS");
> > > + asm volatile ( ".pushsection .text.ex_bras, \"x\"\n"
> > > + "0: bras %[ret_addr],1f\n"
> > > + " nopr %%r7\n"
> > > + "1: larl %[branch_addr],0\n"
> > > + " j 4f\n"
> > > + " .popsection\n"
> > > +
> > > + " larl %[after_target],1b\n"
> > > + " larl %[after_ex],3f\n"
> > > + "2: exrl 0,0b\n"
> /*
> * In case the address calculation is correct, we jump by the relative offset 1b-0b from 0b to 1b.
> * In case the address calculation is relative to the exrl (i.e. a test failure),
> * put a valid instruction at the same relative offset from the exrl, so the test continues in a
> * controlled manner.
> */
looks good
> > > + "3: larl %[branch_addr],0\n"
> > > + "4:\n"
> > > +
> > > + " .if (1b - 0b) != (3b - 2b)\n"
> > > + " .error \"right and wrong target must have same offset\"\n"
> >
> > please explain why briefly (i.e. if the wrong target is executed and
> > the offset mismatches Bad Things™ happen)
>
> Ok, see above.
>
> [...]
>
>
