Re: [PATCH] ext-dce: Avoid multi-register paradoxical subregs for reload [PR122438]

On 7/31/2026 5:35 AM, Karl Mehltretter wrote:
> ext-dce can turn an extension into a paradoxical subreg when the
> upper bits are dead.  With classic reload, this can fail if the modes
> use different numbers of hard registers.  On m68k, SI-to-DI creates an
> invalid overlap and an ICE in postreload.
>
> Skip the rewrite in that case.  This change also fixes PR119953
> on MSP430.  LRA and same-register-count cases are unchanged.
>
> Cross-tested on GCC trunk:
>
> 1) m68k-linux-gnu: the unpatched compiler reproduces PR122438.  The
> gcc.target/m68k suite changes from 128 PASS / 9 FAIL / 1 unsupported
> to 129 PASS / 9 FAIL / 1 unsupported with the new test.
>
> 2) msp430-elf: the new PR119953 test passes.
>
> 3) alpha-linux-gnu and vax-netbsdelf: the tested same-register-count
> extension cases produce identical assembly with and without the patch.
>
> Also tested on GCC 16.1.1 for m68k-linux-gnu.  The unpatched compiler
> reproduces PR122438 while the new test passes with the patch.
>
> Assisted-by: Codex gpt-5.6-terra
>
> gcc/ChangeLog:
>
> 	PR rtl-optimization/122438
> 	PR target/119953
> 	* ext-dce.cc: Include regs.h.
> 	(ext_dce_try_optimize_extension): Avoid multi-register
> 	paradoxical subregs with classic reload.
>
> gcc/testsuite/ChangeLog:
>
> 	PR rtl-optimization/122438
> 	PR target/119953
> 	* gcc.target/m68k/pr122438.c: New test.
> 	* g++.target/msp430/pr119953.C: New test.
This seems like it's just working around bugs elsewhere, particularly 
the targets and/or reload.   But that's a first glance at the patch.

What I would want to see to approve this is an analysis indicating 
how/why the transformed RTL is incorrect.

jeff