[llvm-bugs] [Bug 33605] New: SimplifyCFG destroying canonical loop structure

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=33605

            Bug ID: 33605
           Summary: SimplifyCFG destroying canonical loop structure
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: normal
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: bmakam at codeaurora.org
                CC: llvm-bugs at lists.llvm.org

Created attachment 18718
  --> https://bugs.llvm.org/attachment.cgi?id=18718&action=edit
test.ll

In attached test case, there a loop latch block whose terminator is an
unconditional branch. When simplifyCFG simplifies the latch block it folds it
into the loop header. This destroys the canonical loop structure as it results
in multiple backedges. This prevents later optimizations from being applied
(e.g. loop unroll and loop vectorization). As in this example, LoopSimplify
turns it into a nested loop because there are multiple backedges and now this
loop cannot be unrolled.

The attached testcase(test.ll) is from this c code:
$cat test.c
void foo();
bool test(int a, int b, int *c) {
  bool changed = false;
  for (unsigned int i = 2; i--;) {
    int r = a | b;
    if ( r != c[i]) {
      c[i] = r;
      foo();
      changed = true;
    }
  }
  return changed;
}

With simplifyCFG:

$ opt < test.ll -simplifycfg -loop-rotate -loop-unroll -debug -disable-output
Args: opt -simplifycfg -loop-rotate -loop-unroll -debug -disable-output
Looking to fold if.end into for.cond
Killing Trivial BB:

if.end:                                           ; preds = %if.then, %for.body
  %changed.1.off0 = phi i1 [ true, %if.then ], [ %changed.0.off0, %for.body ]
  br label %for.cond
LoopSimplify: Splitting out a new outer loop
Loop Unroll: F[test] Loop %for.cond
  Loop Size = 9
  will not try to unroll loop with runtime trip count -unroll-runtime not given
Loop Unroll: F[test] Loop %for.cond.outer
  Loop Size = 13
  will not try to unroll loop with runtime trip count -unroll-runtime not given

Without simplifyCFG:

$ opt < new.ll -loop-rotate -loop-unroll -debug -disable-output
Args: opt -loop-rotate -loop-unroll -debug -disable-output
LoopRotation: rotating Loop at depth 1 containing:
%for.cond<header><exiting>,%for.body,%if.then,%if.end<latch>
  Inserted PHI:   %changed.0.off01 = phi i1 [ false, %entry ], [
%changed.0.off0, %for.cond ]
  Inserted PHI:   %dec2 = phi i32 [ 1, %entry ], [ %dec, %for.cond ]
LoopRotation: into Loop at depth 1 containing:
%for.body<header>,%if.then,%if.end<latch><exiting>
Loop Unroll: F[test] Loop %for.body
  Loop Size = 12
  Trip Count = 2
  Trip Multiple = 2
COMPLETELY UNROLLING loop %for.body with trip count 2!
Merging:
for.body.1:                                       ; preds = %if.end
  %or.1 = or i32 %a, %b
  %idxprom.1 = sext i32 %dec to i64
  %arrayidx.1 = getelementptr inbounds i32, i32* %c, i64 %idxprom.1
  %1 = load i32, i32* %arrayidx.1, align 4
  %cmp.1 = icmp eq i32 %or.1, %1
  br i1 %cmp.1, label %if.end.1, label %if.then.1
into:
if.end:                                           ; preds = %if.then, %for.body
  %changed.1.off0 = phi i1 [ true, %if.then ], [ false, %for.body ]
  %dec = add nsw i32 1, -1
  %tobool = icmp eq i32 1, 0
  br label %for.body.1
Merging:
for.cond.cleanup:                                 ; preds = %if.end.1
  %changed.0.off0.lcssa = phi i1 [ %changed.1.off0.1, %if.end.1 ]
  ret i1 %changed.0.off0.lcssa
into:
if.end.1:                                         ; preds = %if.then.1, %if.end
  %changed.1.off0.1 = phi i1 [ true, %if.then.1 ], [ %changed.1.off0, %if.end ]
  %dec.1 = add nsw i32 %dec, -1
  %tobool.1 = icmp eq i32 %dec, 0
  br label %for.cond.cleanup

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