https://gcc.gnu.org/bugzilla/show_bug.cgi?id=116542
Bug ID: 116542
Summary: [avr] Missed post increment optimization
Product: gcc
Version: 14.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: gjl at gcc dot gnu.org
Target Milestone: ---
Created attachment 59027
--> https://gcc.gnu.org/bugzilla/attachment.cgi?id=59027&action=edit
incdec.c: GNU-C99 test case
The attached test case produces very expensive code. For example the following
function (with decent resource consumption as comments):
$ avr-gcc incdec.c -mmcu=avr4 -S -Os -dp
uint8_t add1 (const uint8_t *bb, uint8_t nn)
{
// Set Z or X = bb (1 MOVW or 2 MOVs)
uint8_t sum = 0; // 1 instruction
do
{
sum += *bb++; // 2 instructions: POST_INC load + add
} while (--nn); // 2 instructions: decrement + branch
return sum;
}
add1:
/* prologue: function */
/* frame size = 0 */
/* stack size = 0 */
.L__stack_usage = 0
mov r20,r24 ; 47 [c=4 l=1] movqi_insn/0
mov r18,r24 ; 48 [c=4 l=1] movqi_insn/0
mov r19,r25 ; 49 [c=4 l=1] movqi_insn/0
ldi r24,0 ; 50 [c=4 l=1] movqi_insn/0
.L5:
movw r30,r18 ; 38 [c=4 l=1] *movhi/0
subi r18,-1 ; 39 [c=4 l=2] *addhi3_clobber/1
sbci r19,-1
ld r25,Z ; 40 [c=4 l=1] movqi_insn/3
add r24,r25 ; 41 [c=4 l=1] *addqi3/0
mov r25,r22 ; 42 [c=4 l=1] movqi_insn/0
sub r25,r18 ; 43 [c=4 l=1] *subqi3/0
add r25,r20 ; 55 [c=4 l=1] *op8.for.cczn.plus/1
brne .L5 ; 56 [c=4 l=1] branch_ZN
/* epilogue start */
ret ; 53 [c=0 l=1] return
The loop body uses R18 to hold and compute the address and R30 (Z) for
accessing, instead of just R30 with post-increment. (LD reg, Z+).
Then there is this fancy loop invariant of start_address - current_address + nn
== 0 instead of just decrementing nn (-fivopts issue / assumptions I guess).
Target: avr
Configured with: ../../source/gcc-master/configure --target=avr --disable-nls
--with-dwarf2 --with-gnu-as --with-gnu-ld --disable-shared
--with-long-double=64 --enable-languages=c,c++
gcc version 15.0.0 20240829 (experimental) (GCC)
