https://gcc.gnu.org/bugzilla/show_bug.cgi?id=114100
Bug ID: 114100
Summary: [avr] Inefficient indirect addressing on Reduced Tiny
Product: gcc
Version: 13.2.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: target
Assignee: unassigned at gcc dot gnu.org
Reporter: gjl at gcc dot gnu.org
Target Milestone: ---
The Reduced Tiny core does not support indirect addressing with offset, which
basically means that every indirect memory access with a size of more than one
byte is effectively POST_INC or PRE_DEC. The lack of that addressing mode is
currently handled by pretending to support it, and then let the insn printers
add and subtract again offsets as needed on the fly.
For example, the following C code
int vars[10];
void inc_var2 (void) {
++vars[2];
}
is compiled to:
ldi r30,lo8(vars) ; 14 [c=4 l=2] *movhi/4
ldi r31,hi8(vars)
subi r30,lo8(-(4)) ; 15 [c=8 l=6] *movhi/2
sbci r31,hi8(-(4))
ld r20,Z+
ld r21,Z
subi r30,lo8((4+1))
sbci r31,hi8((4+1))
subi r20,-1 ; 16 [c=4 l=2] *addhi3_clobber/1
sbci r21,-1
subi r30,lo8(-(4+1)) ; 17 [c=4 l=4] *movhi/3
sbci r31,hi8(-(4+1))
st Z,r21
st -Z,r20
where the code could be:
ldi r30,lo8(vars+4) ; 28 [c=4 l=2] *movhi/4
ldi r31,hi8(vars+4)
ld r20,Z+ ; 17 [c=8 l=2] *movhi/2
ld r21,Z+
subi r20,-1 ; 19 [c=4 l=2] *addhi3_clobber/1
sbci r21,-1
st -Z,r21 ; 30 [c=4 l=2] *movhi/3
st -Z,r20
