Re: [PATCH] x86: Generate .cfi_undefined for unsaved callee-saved registers

["H.J. Lu" <hjl.tools@gmail.com>]

On Mon, Jan 29, 2024 at 2:08 AM Jakub Jelinek <jakub@redhat.com> wrote:
>
> On Sat, Jan 27, 2024 at 12:41:24PM -0800, H.J. Lu wrote:
> > When assembler directives for DWARF frame unwind is enabled, generate
> > the .cfi_undefined directive for unsaved callee-saved registers which
> > have been used in the function.
> >
> > gcc/
> >
> >       PR target/38534
> >       * config/i386/i386.cc (ix86_post_cfi_startproc): New.
> >       (TARGET_ASM_POST_CFI_STARTPROC): Likewise.
> >
> > gcc/testsuite/
> >
> >       PR target/38534
> >       * gcc.target/i386/no-callee-saved-19.c: New test.
> >       * gcc.target/i386/no-callee-saved-20.c: Likewise.
> >       * gcc.target/i386/pr38534-7.c: Likewise.
> >       * gcc.target/i386/pr38534-8.c: Likewise.
>
> This only works for -fdwarf2-cfi-asm, but doesn't work for
> -fno-dwarf2-cfi-asm.  I think we need something that will work for both.

-fno-dwarf2-cfi-asm stops generating all CFI directives.

> So, I'd say we want to add support for REG_CFA_UNDEFINED note, emit those
> notes on some frame related insn in the prologue during prologue expansion
> in pro_and_epilogue pass and handle that in dwarf2cfi.cc pass.

It is a good idea.  Here is the patch:

https://patchwork.sourceware.org/project/gcc/list/?series=30314

> One question is where those should be emitted.  Emitting them right
> at the start of the function has an advantage that it can be emitted in
> CIE for all FDEs of noreturn functions (or with the new attribute).  But
> disadvantage is of course that it will make e.g. debugging experience worse
> even in the prologues of functions where the callee saved registers which
> current function actually doesn't save aren't modified yet.
> E.g. for the cases where callee saved registers are saved to memory or
> registers I think dwarf2cfi.cc attempts to optimize and move the .cfi_*
> directives or .eh_frame record later into the function as long as the
> corresponding original register isn't modified yet.  Perhaps that should
> be done also for the undefined case, ideally by using the same dwarf2cfi.cc
> code.  So just perhaps at the start of the function read in the
> REG_CFA_UNDEFINED notes for all the ever modified callee saved registers
> which won't be actually saved and turn that into similar record like for
> the saving into stack or other regs, just noting it is undefined instead
> and have it pushed later as much as possible.

My patch doesn't implement this optimization.

-- 
H.J.