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

[Jakub Jelinek <jakub@redhat.com>]

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.

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.

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.

	Jakub