https://llvm.org/bugs/show_bug.cgi?id=25507
Bug ID: 25507
Summary: [Polly] Unnecessarily complicated schedule
Product: Projects
Version: unspecified
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Polly
Assignee: polly-...@googlegroups.com
Reporter: l...@meinersbur.de
CC: llvm-bugs@lists.llvm.org
Classification: Unclassified
Created attachment 15279
--> https://llvm.org/bugs/attachment.cgi?id=15279&action=edit
Extract from MultiSource/Benchmark/Ptrdist/bc/number.c
$ clang loopschedule.c -c -O3 -mllvm -polly -mllvm -debug -mllvm
-debug-only=polly-ast -mllvm -polly-process-unprofitable
emits the following schedule for the loop "while.body => while.end"
for (int c0 = 0; c0 < min(n2, n1); c0 += 1)
Stmt_while_body(c0);
if (n1 <= 0) {
Stmt_while_body(0);
} else if (n2 <= 0)
Stmt_while_body(0);
Context:
[n2, n1] -> { : n2 >= -2147483648 and n2 <= 2147483647 and n1 >=
-2147483648 and n1 <= 2147483647 }
Assumed Context:
[n2, n1] -> { : }
Boundary Context:
[n2, n1] -> { : }
Statements
Stmt_while_body
Domain :=
[n2, n1] -> { Stmt_while_body[i0] : i0 >= 0 and i0 <= -1 + n1
and i0 <= -1 + n2; Stmt_while_body[0] : n1 <= 0 or (n2 <= 0 and n1 >= 1) };
The initial loop condition is hoisted outside of the scop, i.e
if ((n1 > 0) && (n2 > 0)) {
// Scop
}
1) The generated schedule has two conditions (n1 <= 0) and (n2 <= 0) that are
never evaluated code, i.e. 2/3 of the generated code is effectively dead.
2) After passing the hoisted initial condition, the BasicBlock while.body must
be executed at least once before the condition is checked again. Still, CodeGen
inserts a second initial check into "polly.loop_if". It results into an
additional branch that is not optimized away by later LLVM passes and
potentially "undef" to appear after -mem2reg.
We might analyze the hoisted condition to translate into additional assumptions
(Here: n1 > 0 && n2 > 0). Unfortunately, there is no LLVM analysis to tell the
the possible range of a value.
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