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From: "simon.willcocks at gmx dot de" <gcc-bugzilla@gcc.gnu.org>
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Subject: [Bug c/101422] New: register variable uninitialised before passing
 to asm
Date: Sun, 11 Jul 2021 18:23:50 +0000
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https://gcc.gnu.org/bugzilla/show_bug.cgi?id=3D101422

            Bug ID: 101422
           Summary: register variable uninitialised before passing to asm
           Product: gcc
           Version: 10.3.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: c
          Assignee: unassigned at gcc dot gnu.org
          Reporter: simon.willcocks at gmx dot de
  Target Milestone: ---

Created attachment 51132
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=3D51132&action=3Dedit
Source code fragment

The attached program fragment sets up multiple register variables associated
with specific ARM registers and runs some assembly code that makes use of t=
hem.
With -Os (and a generated memset to initialise a local array), some registe=
rs
are not correctly initialised, notably r0, which is corrupted by the addres=
s of
the array, and r3 which is not initialised at all before the asm instructio=
n.
r4, however, is correctly initialised.

Affects arm-linux-gnueabi-gcc-10, arm-linux-gnueabi-gcc-9, and
arm-linux-gnueabi-gcc-8.

for opt in 1 2 3 4 s ; do for vers in 8 9 10 ; do arm-linux-gnueabi-gcc-$ve=
rs
problem.c -S -O$opt -Wall -Wextra -o problem-$vers-$opt.s ; done ; done

To see the problem (the most obvious feature, at least):

for i in problem*.s ; do echo ; echo $i ; grep r3 $i ; done

In all three versions with -Os, the first mention of r3 is the asm code.=