On Tue, May 12, 2026 at 3:00 PM Uros Bizjak <[email protected]> wrote:
>
> On Tue, May 12, 2026 at 8:47 AM H.J. Lu <[email protected]> wrote:
> >
> > On Tue, May 12, 2026 at 2:39 PM Uros Bizjak <[email protected]> wrote:
> > >
> > > On Tue, May 12, 2026 at 2:42 AM H.J. Lu <[email protected]> wrote:
> > > >
> > > > DRAP register is used to restore stack pointer in epilogue for stack
> > > > realignment. In no-callee-saved and preserve_none functions, although
> > > > DRAP register isn't preserved, it must be unchanged between prologue
> > > > and epilogue so that stack pointer can be restored. In no-callee-saved
> > > > and preserve_none functions, mark BX_REG as fixed and use it for DRAP
> > > > register.
> > > >
> > > > NB: r12-r15 aren't made available as the local general purpose registers
> > > > since it causes:
> > > >
> > > > FAIL: gcc.target/i386/preserve-none-1.c scan-assembler-not movq
> > > > FAIL: gcc.target/i386/preserve-none-30a.c check-function-bodies entry
> > > > FAIL: gcc.target/i386/preserve-none-30b.c check-function-bodies entry
> > > >
> > > > which have the same cause as
> > > >
> > > > https://gcc.gnu.org/bugzilla/show_bug.cgi?id=124798
> > > >
> > > > gcc/
> > > >
> > > > PR target/120870
> > > > * config/i386/i386.cc (X86_NO_CALLEE_DRAP_REG): New.
> > > > (ix86_conditional_register_usage): Mark X86_NO_CALLEE_DRAP_REG
> > > > as fixed.
> > > > (find_drap_reg): Return X86_PRESERVE_NONE_DRAP_REG for
> > > > no-callee-saved and preserve_none functions
> > > >
> > > > gcc/testsuite/
> > > >
> > > > PR target/120870
> > > > * gcc.target/i386/pr120870-1.c: New test.
> > > > * gcc.target/i386/pr120870-2.c: Likewise.
> > >
> > > + case TYPE_NO_CALLEE_SAVED_REGISTERS:
> > > + case TYPE_PRESERVE_NONE:
> > > + /* DRAP register is used to restore stack pointer in epilogue
> > > + for stack realignment. Although DRAP register isn't
> > > + preserved, it must be unchanged between prologue and
> > > + epilogue so that stack pointer can be restored. In
> > > + no-callee-saved and preserve_none function, mark BX_REG
> > > + as fixed and use it for DRAP register. */
> > > + fixed_regs[X86_NO_CALLEE_DRAP_REG] = 1;
> > >
> > > Isn't this hammer too big? This will remove RBX even when DRAP is not
> > > needed, which I assume is in the majority of cases.
> > >
> >
> > It is only for preserve_none functions where all, but BX, SP, FP, are
> > scratch registers. It should be OK.
>
> But your patch returns X86_NO_CALLEE_DRAP_REG (AKA RBX) from
> find_drap_reg. This should signal RA that the register is unavailable.
> Why doesn't it?
>
True, but only for no-callee-saved and preserve_none function:
static unsigned int
find_drap_reg (void)
{
if (cfun->machine->call_saved_registers == TYPE_PRESERVE_NONE
|| (cfun->machine->call_saved_registers
== TYPE_NO_CALLEE_SAVED_REGISTERS))
return X86_NO_CALLEE_DRAP_REG;
--
H.J.
