optimization/10842: [arm] Clobbered link register is copied to pc under certain circumstances

[zhenya at silverbrookresearch dot com]

>Number:         10842
>Category:       optimization
>Synopsis:       [arm] Clobbered link register is copied to pc under certain circumstances
>Confidential:   no
>Severity:       serious
>Priority:       medium
>Responsible:    unassigned
>State:          open
>Class:          wrong-code
>Submitter-Id:   net
>Arrival-Date:   Sun May 18 06:26:00 UTC 2003
>Closed-Date:
>Last-Modified:
>Originator:     Zhenya Yourlo
>Release:        gcc version 3.3
>Organization:
>Environment:
Host: FreeBSD 4.7-STABLE
Target: arm-elf
Configured with: ../configure --host=i386-unknown-freebsd4.7 --program-prefix=arm-elf- --target=arm-elf --disable-threads --with-cpu=arm7tdmi --enable-languages=c
>Description:
Compiling the attached code with -O3 -fno-inline-functions produces assembly that exhibits the following behaviour:

functionA branches with link to functionB.
functionB clobbers the link register and branches to functionC.
functionC returns with "mov pc, lr" but lr is no longer valid.

When compiling with -O2, the link register is not used as a scratch register in functionB, and hence the "mov pc, lr" used by functionC works as expected.

This appears to be a regression from 3.2.1 (has not been tested with 3.2.2).
>How-To-Repeat:
arm-elf-gcc -O3 -fno-inline-functions -save-temps -c gcc33bug.i
>Fix:
A workaround is to use -O2 in place of -O3, or use -ffixed-lr
>Release-Note:
>Audit-Trail:
>Unformatted:
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