https://gcc.gnu.org/bugzilla/show_bug.cgi?id=65326
Bug ID: 65326
Summary: LRA missing a Thumb optimization.
Product: gcc
Version: 4.9.3
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: matthew.wahab at arm dot com
Created attachment 34964
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=34964&action=edit
Testcase showing change in behaviour.
The ARM backend no longer supports -mno-lra so only the LRA is available. This
has also removed the Thumb mode optimiziation introduced in
https://gcc.gnu.org/ml/gcc-patches/2005-08/msg01140.html to fix PR 23436.
This turns sequences like
mov r3, r9
mov r2, r10
ldr r0, [r3, r2]
into
mov r3, r9
add r3, r3, r10
ldr r0, [r3]
which saves a register.
Attached is a contrived test case. Compiling with gcc-4.9 with -mthumb -mno-lra
(at -O1 and higher) produces the second (better) sequence. Compiling with
gcc-4.9 or gcc-trunk with -mthumb (at -O1 and higher) produces the first
sequence. The sequences appear after the 'nop'
gcc-4.9 is
arm-none-eabi-gcc (GNU Tools for ARM Embedded Processors) 4.9.3 20141119
(release) [ARM/embedded-4_9-branch revision 218278]
trunk is:
arm-none-eabi-gcc (unknown) 5.0.0 20150217 (experimental)
