The result of a signed remainder operation with a constant divisor is limited
in absolute value to less than the value of the divisor. Following it with
code to force the remainder to be positive by adjusting the quotient and
remainder values is pretty straightforward. However, if it's written as a loop
it doesn't get optimized well.
The rtl initially generated appears to have the loop transformed into
arithmetic to figure out the number of times the loop would run, in a branch
conditionalized on whether the loop would be run at all. (Actually, it appears
to run the loop body once, and then do math to figure out how many more times.)
However, the compiler doesn't figure out that in that branch, the loop body
would be run exactly once, either in the tree or rtl optimizations.
--
Summary: limited range of remainder operation can prove loop runs
at most once
Product: gcc
Version: unknown
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
AssignedTo: unassigned at gcc dot gnu dot org
ReportedBy: raeburn at raeburn dot org
GCC build triplet: i686-pc-linux-gnu
GCC host triplet: i686-pc-linux-gnu
GCC target triplet: i686-pc-linux-gnu
http://gcc.gnu.org/bugzilla/show_bug.cgi?id=33259
