[PATCH] PR rtl-optimization/66790: uninitialized registers handling in REE

[Pierre-Marie de Rodat]

Hello,

This patch is an attempt to fix PR rtl-optimization/66790: please see 
<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=66790> for the context. 
This adds a new dataflow problem (MIR for Must-Initialized Registers) 
and use it in the REE pass to remove oversights, fixing the original issue.

I would also like to draw your attention to the use of the 
"must-initialized" term. Being new to dataflow problems, I understand it 
as "registers that are always initialized whatever the path leading to 
its use" and this seems to be confirmed by the current "LIVE AND 
MUST-INITIALIZED REGISTERS" comment in df-problems.c. However, it feels 
like what is currently called "must-initialized" in this source file is 
actually registers that *may* be initialized: see in particular the use 
of the bitmap_ior_into operation in df_live_confluence_n.

Can someone confirm this, please? If I'm right, I will update the 
attached patch to reword the corresponding comments.

This patch bootstraps and yields no regression on x86_64-linux. I'm 
still trying to fiddle the build system to actually see how assemblies 
from the bootstrap are affected. Thank you in advance for the review! :-)

gcc/ChangeLog:

         PR rtl-optimization/66790
         * df.h (DF_MIR): New macro.
         (DF_LAST_PROBLEM_PLUS1): Update to be past DF_MIR
         (DF_MIR_INFO_BB): New macro.
         (DF_MIR_IN, DF_MIR_OUT): New macros.
         (struct df_mir_bb_info): New.
         (df_mir): New macro.
         (df_mir_add_problem, df_mir_simulate_one_insn): New forward
         declarations.
         (df_mir_get_bb_info): New.
         * df-problems.c (struct df_mir_problem_data): New.
         (df_mir_free_bb_info, df_mir_alloc, df_mir_reset,
         df_mir_bb_local_compute, df_mir_local_compute, df_mir_init,
         df_mir_confluence_n, df_mir_transfer_function, df_mir_free,
         df_mir_top_dump, df_mir_bottom_dump,
         df_mir_verify_solution_start, df_mir_verify_solution_end): New.
         (problem_MIR): New.
         (df_mir_add_problem, df_mir_simulate_one_insn): New.
         * timevar.def (TV_DF_MIR): New.
         * ree.c: Include bitmap.h
         (add_removable_extension): Add an INIT_REGS parameter.  Use it
         to skip extensions that may get an uninitialized register.
         (find_removable_extensions): Compute must-initialized registers
         using the MIR dataflow problem. Update the call to
         add_removable_extension.
         (find_and_remove_re): Call df_mir_add_problem.

-- 
Pierre-Marie de Rodat
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