http://gcc.gnu.org/bugzilla/show_bug.cgi?id=53265
Bug #: 53265
Summary: Warn when undefined behavior implies smaller iteration
count
Classification: Unclassified
Product: gcc
Version: 4.8.0
Status: UNCONFIRMED
Keywords: diagnostic
Severity: enhancement
Priority: P3
Component: tree-optimization
AssignedTo: [email protected]
ReportedBy: [email protected]
As requested in PR 53128, this is an enhancement request that asks to produce a
warning when loop bound inferred from undefined behavior may be used to
transform the loop in a surprising way.
Example code:
enum {N=4};
int a[N], pfx[N];
void foo()
{
int i, accum;
for (i=0, accum=a[0]; i < N; i++, accum+=a[i])
pfx[i] = accum;
}
VRP in trunk produces an infinite loop from the above. By the way, with
-fno-tree-vrp one would expect that cunroll unrolls just 3 iterations of the
loop, but in fact it unrolls 4, because nb_iterations is not updated when
nb_iterations_upper_bound is reduced from UB analysis.
Can we simply warn when UB analysis reduces nb_iterations_upper_bound and makes
it statically smaller than symbolic nb_iterations?
