https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102893
Bug ID: 102893
Summary: [8/9/10/11/12 Regression] CDDCE does not detect empty
infinite nested loops
Product: gcc
Version: 8.5.0
Status: UNCONFIRMED
Keywords: missed-optimization
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: tnfchris at gcc dot gnu.org
Target Milestone: ---
The following example
int main() {
while(1) {
for(int i=0; i<9000000; i++){}
}
}
In GCC 7 compiled to
main:
.L2:
b .L2
but since GCC 8 compiles to
main:
.L3:
mov w0, 21568
movk w0, 0x89, lsl 16
.L2:
subs w0, w0, #1
bne .L2
b .L3
The difference seems to be in cddce which previously was able to detect the
code as dead:
;; 3 loops found
;;
;; Loop 0
;; header 0, latch 1
;; depth 0, outer -1
;; nodes: 0 1 2 3 4 5 6
;;
;; Loop 1
;; header 3, latch 6
;; depth 1, outer 0
;; nodes: 3 6 5 4
;;
;; Loop 2
;; header 5, latch 4
;; depth 2, outer 1
;; nodes: 5 4
;; 2 succs { 3 }
;; 3 succs { 5 }
;; 4 succs { 5 }
;; 5 succs { 4 6 }
;; 6 succs { 3 }
can not prove finiteness of loop 1
Removing basic block 4
Merging blocks 3 and 5
Removing basic block 6
but since GCC 8 does
;; 3 loops found
;;
;; Loop 0
;; header 0, latch 1
;; depth 0, outer -1
;; nodes: 0 1 2 3 4 5 6
;;
;; Loop 1
;; header 3, latch 6
;; depth 1, outer 0
;; nodes: 3 6 5 4
;;
;; Loop 2
;; header 5, latch 4
;; depth 2, outer 1
;; nodes: 5 4
;; 2 succs { 3 }
;; 3 succs { 5 }
;; 4 succs { 5 }
;; 5 succs { 4 6 }
;; 6 succs { 3 }
cannot prove finiteness of loop 1
