[RFC] More jump threading restrictions in the presence of loops.

[Andreas Schwab]

On Okt 05 2021, Aldy Hernandez via Gcc-patches wrote:

> From 5abe65668f602d53b8f3dc515508dc4616beb048 Mon Sep 17 00:00:00 2001
> From: Aldy Hernandez <aldyh@redhat.com>
> Date: Tue, 5 Oct 2021 15:03:34 +0200
> Subject: [PATCH] Loosen loop crossing restriction in threader.
>
> Crossing loops is generally discouraged from the threader, but we can
> make an exception when we don't cross the latch or enter another loop,
> since this is just an early exit out of the loop.

This breaks bootstrap on aarch64 (in stage2):

In function 'void mark_stack_region_used(poly_uint64, poly_uint64)',
    inlined from 'rtx_def* emit_library_call_value_1(int, rtx, rtx, libcall_type, machine_mode, int, rtx_mode_t*)' at ../../gcc/calls.c:4536:29:
../../gcc/calls.c:206:26: error: 'const_upper' may be used uninitialized in this function [-Werror=maybe-uninitialized]
  206 |       stack_usage_map[i] = 1;
      |       ~~~~~~~~~~~~~~~~~~~^~~
../../gcc/calls.c: In function 'rtx_def* emit_library_call_value_1(int, rtx, rtx, libcall_type, machine_mode, int, rtx_mode_t*)':
../../gcc/calls.c:202:39: note: 'const_upper' was declared here
  202 |   unsigned HOST_WIDE_INT const_lower, const_upper;
      |                                       ^~~~~~~~~~~

Andreas.

-- 
Andreas Schwab, schwab@linux-m68k.org
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