https://gcc.gnu.org/bugzilla/show_bug.cgi?id=64760
Bug ID: 64760
Summary: [SH] Avoid multiple #imm,r0 insns with the same #imm
value
Product: gcc
Version: 5.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: olegendo at gcc dot gnu.org
Target: sh*-*-*
In libstdc++ there is code such as:
mov r9,r0
cmp/eq #-1,r0
mov r1,r0
movt r2
cmp/eq #-1,r0
movt r1
cmp/eq r1,r2
bt ...
It seems that it's better to avoid #imm,r0 insns such as cmp/eq, and, or, xor,
tst, if the #imm value can be shared among several insns. The insns don't have
to be same, only the constant value.
The above code could be something like:
mov #-1,r0
cmp/eq r9,r0
movt r2
cmp/eq r1,r0
movt r1
cmp/eq r1,r2
Preferably, this should be done before RA to reduce r0 pressure. It can be
accomplished by simply loading the constant into a pseudo and replacing the
operands in the insns.
On the other hand, if the other operand (other than #imm) is likely to end up
in r0, the #imm,r0 insn is probably going to be better.
A possible metric for 'likely to be in r0' could be:
- the operand is loaded using a mov.{b|w} @(disp,Rn),R0
- the operand is needed in r0 by some other surrounding insns
