Take the following simple source:
int X[4] = {1};
int *foo() { return &X[1]; }
----
With -static -O2, we get:
_foo:
lis r3,ha16(_X+4)
la r3,lo16(_X+4)(r3)
blr
---
Which is the most optimial.
With -O2 -mdynamic-no-pic, we get:
_foo:
lis r3,ha16(_X)
la r3,lo16(_X)(r3)
addi r3,r3,4
blr
Which is not optimial at all.
The PIC version is just as bad:
_foo:
mflr r0
bcl 20,31,"L00000000001$pb"
"L00000000001$pb":
stw r31,-4(r1)
mflr r31
mtlr r0
addis r3,r31,ha16(_X-"L00000000001$pb")
lwz r31,-4(r1)
la r3,lo16(_X-"L00000000001$pb")(r3)
addi r3,r3,4
blr
-----
All of the ha16 and lo16 should be including +4 in them instead of explicatly
adding 4 to the register.
This testcase comes from Chris Lattner.
--
Summary: CONST (PLUS a + 4) is not done for -fPIC or -mdynamic-
no-pic
Product: gcc
Version: 4.1.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: target
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: pinskia at gcc dot gnu dot org
GCC target triplet: powerpc-darwin
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=24910
