[llvm] [LoopPeel] LCSSA form is destroyed by LoopPeel, preserve it (PR #78696)
Nikita Popov via llvm-commits
llvm-commits at lists.llvm.org
Fri Feb 16 06:49:29 PST 2024
nikic wrote:
@vedantparanjape-amd If the LCSSA of the parent loop is already destroyed by the time we enter LoopPeel, then I don't think trying to recover it inside LoopPeel is the right thing to do. We should fix this at the point where LCSSA is broken.
Do I understand correctly that the only relation LoopPeel has to anything here is that it contains this LCSSA assertion? It looks like adding
```
for (Loop *L : LI)
assert(L->isRecursivelyLCSSAForm(DT, LI));
```
to the end of unrolling triggers the assertion even without peeling.
-----
For the record, here is the test case with cleaned up block naming:
```llvm
target triple = "x86_64-unknown-linux-gnu"
define void @test() !prof !0 {
entry:
br label %loop1
loop1:
%phi = phi i32 [ 1, %entry ], [ 0, %loop1.latch ]
br label %loop2
loop2:
%phi3 = phi i64 [ 0, %loop1 ], [ %sext, %loop2.latch ]
br label %loop3
loop3:
%phi5 = phi i64 [ %phi3, %loop2 ], [ %sext, %loop3.latch ]
%phi6 = phi i32 [ 1, %loop2 ], [ %add10, %loop3.latch ]
%trunc = trunc i64 %phi5 to i32
br i1 true, label %loop3.latch, label %exit
loop3.latch:
%add = add i32 1, %phi
%sext = sext i32 %add to i64
%add10 = add i32 %phi6, 1
%icmp = icmp ugt i32 %add10, 2
br i1 %icmp, label %loop2.latch, label %loop3
loop2.latch:
br i1 false, label %loop4.preheader, label %loop2
loop4.preheader:
br label %loop4
loop4:
br i1 false, label %loop1.latch, label %loop4
loop1.latch:
br label %loop1
exit:
%phi8 = phi i32 [ %trunc, %loop3 ]
ret void
}
!0 = !{!"function_entry_count", i64 32768}
!1 = !{!"branch_weights", i32 1, i32 1}
```
https://github.com/llvm/llvm-project/pull/78696
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