https://bugs.llvm.org/show_bug.cgi?id=34171
Bug ID: 34171
Summary: Simple no-op loops with a termination flag are not
optimized out
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Loop Optimizer
Assignee: unassignedb...@nondot.org
Reporter: geoffrey@allott.email
CC: llvm-bugs@lists.llvm.org
The following C code results in a loop with many iterations even with -O3; I
would expect it to be optimized out:
------------------------
void do_nothing() {
int i=200, q=0;
while(!q) if (!--i) i=q=1;
}
------------------------
; Function Attrs: norecurse nounwind readnone sspstrong uwtable
define void @do_nothing() local_unnamed_addr #0 {
br label %1
; <label>:1: ; preds = %1, %0
%2 = phi i32 [ 200, %0 ], [ %3, %1 ]
%3 = add nsw i32 %2, -25
%4 = icmp eq i32 %3, 0
br i1 %4, label %5, label %1
; <label>:5: ; preds = %1
ret void
}
------------------------
movl $-200, %eax
.LBB0_1:
addl $25, %eax
jne .LBB0_1
retq
------------------------
(Any number low enough seems to optimize out - I assume because of some
hardcoded limits that simply evaluate some number of loops.)
It seems as though the problem is the fact that `i` is set to 1 on exit of the
loop - if this is removed then the loop is optimized away no matter the inital
value of i.
This situation can come up very often whenever a flag is used as the condition
to exit a loop. For instance, no-op loops in rust are not optimized out by llvm
because they terminate in such a way.
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