[llvm-bugs] [Bug 32998] New: [Polly - AST Generation] Derive knowledge about loops that are known to be executed at least once
via llvm-bugs
llvm-bugs at lists.llvm.org
Wed May 10 23:39:50 PDT 2017
https://bugs.llvm.org/show_bug.cgi?id=32998
Bug ID: 32998
Summary: [Polly - AST Generation] Derive knowledge about loops
that are known to be executed at least once
Product: Polly
Version: unspecified
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: isl
Assignee: polly-dev at googlegroups.com
Reporter: tobias at grosser.es
CC: llvm-bugs at lists.llvm.org
Polly generates in lib/CodeGen/LoopGenerators.cpp:createLoop one of the two
loop forms:
// BeforeBB BeforeBB
// | |
// v v
// GuardBB PreHeaderBB
// / | | _____
// __ PreHeaderBB | v \/ |
// / \ / | HeaderBB latch
// latch HeaderBB | |\ |
// \ / \ / | \------/
// < \ / |
// \ / v
// ExitBB ExitBB
In C, this means a loop:
for (i = LB; i <= UB; i++)
S(i);
is generated either as:
if (LB <= UB) {
long i = LB;
do {
S(i);
i++;
} while (LB <= UB);
}
or as
long i = LB;
do {
S(i);
i++;
} while (LB <= UB);
in case we can statically proof that LB <= UB. Polly can do this only for
integer constants, but isl can likely proof this information a lot more often.
It would be great if isl can provide this information, such that we can avoid
code size increase due to the unnecessary condition. The condition likely also
hinders loop invariant code motion.
Background: Polly (and LLVM in general) does this rewriting to move branches
after the loop body, such that the content of the loop body can be hoisted out
of the loop without any control flow branches blocking this hoisting.
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