Revert 250343 and 250344

Turns out this approach is buggy.  In discussion about follow on work, Sanjoy pointed out that we could be subject to circular logic problems.  

Consider:
 if (i u< L) leave()
 if ((i + 1) u< L) leave()
 print(a[i] + a[i+1]) 

If we know that L is less than UINT_MAX, we could possible prove (in a control dependent way) that i + 1 does not overflow.  This gives us:
 if (i u< L) leave()
 if ((i +nuw 1) u< L) leave()
 print(a[i] + a[i+1]) 

If we now do the transform this patch proposed, we end up with:
 if ((i +nuw 1) u< L) leave_appropriately()
 print(a[i] + a[i+1]) 

That would be a miscompile when i==-1.  The problem here is that the control dependent nuw bits got used to prove something about the first condition.  That's obviously invalid.

This won't happen today, but since I plan to enhance LVI/CVP with exactly that transform at some point in the not too distant future...

llvm-svn: 250430

1 files changed, 0 insertions, 86 deletions

diff --git a/llvm/test/Transforms/SimplifyCFG/fast-fallthrough.ll b/llvm/test/Transforms/SimplifyCFG/fast-fallthrough.ll
deleted file mode 100644
index 7c5dbe2b0ad..00000000000
--- a/llvm/test/Transforms/SimplifyCFG/fast-fallthrough.ll
+++ /dev/null
@@ -1,86 +0,0 @@
-; RUN: opt -S %s -simplifycfg | FileCheck %s
-
-define void @test(i32 %length.i, i32 %i) {
-; CHECK-LABEL: @test
-  %iplus1 = add nsw i32 %i, 1
-  %var29 = icmp slt i32 %i, %length.i
-  %var30 = icmp slt i32 %iplus1, %length.i
-; CHECK: br i1 %var30, label %in_bounds, label %next
-  br i1 %var29, label %next, label %out_of_bounds, !prof !{!"branch_weights", i32 1000, i32 0}
-
-next:
-; CHECK-LABEL: next:
-; CHECK: br i1 %var29, label %out_of_bounds2, label %out_of_bounds
-  br i1 %var30, label %in_bounds, label %out_of_bounds2, !prof !{!"branch_weights", i32 1000, i32 0}
-
-in_bounds:
-  ret void
-
-out_of_bounds:
-  call void @foo(i64 0)
-  unreachable
-
-out_of_bounds2:
-  call void @foo(i64 1)
-  unreachable
-}
-
-define void @test2(i32 %length.i, i32 %i) {
-; CHECK-LABEL: @test2
-  %var29 = icmp slt i32 %i, %length.i
-; CHECK: br i1 %var30, label %in_bounds, label %next
-  br i1 %var29, label %next, label %out_of_bounds, !prof !{!"branch_weights", i32 1000, i32 0}
-
-next:
-; CHECK-LABEL: next:
-; CHECK: br i1 %var29, label %out_of_bounds2, label %out_of_bounds
-  %iplus1 = add nsw i32 %i, 1
-  %var30 = icmp slt i32 %iplus1, %length.i
-  br i1 %var30, label %in_bounds, label %out_of_bounds2, !prof !{!"branch_weights", i32 1000, i32 0}
-
-in_bounds:
-  ret void
-
-out_of_bounds:
-  call void @foo(i64 0)
-  unreachable
-
-out_of_bounds2:
-  call void @foo(i64 1)
-  unreachable
-}
-
-; As written, this one can't trigger today.  It would require us to duplicate
-; the %val1 load down two paths and that's not implemented yet.
-define i64 @test3(i32 %length.i, i32 %i, i64* %base) {
-; CHECK-LABEL: @test3
-  %var29 = icmp slt i32 %i, %length.i
-; CHECK: br i1 %var29, label %next, label %out_of_bounds
-  br i1 %var29, label %next, label %out_of_bounds, !prof !{!"branch_weights", i32 1000, i32 0}
-
-next:
-; CHECK-LABEL: next:
-  %addr1 = getelementptr i64, i64* %base, i32 %i
-  %val1 = load i64, i64* %addr1
-  %iplus1 = add nsw i32 %i, 1
-  %var30 = icmp slt i32 %iplus1, %length.i
-; CHECK: br i1 %var30, label %in_bounds, label %out_of_bounds2
-  br i1 %var30, label %in_bounds, label %out_of_bounds2, !prof !{!"branch_weights", i32 1000, i32 0}
-
-in_bounds:
-  %addr2 = getelementptr i64, i64* %base, i32 %iplus1
-  %val2 = load i64, i64* %addr2
-  %res = sub i64 %val1, %val2
-  ret i64 %res
-
-out_of_bounds:
-  call void @foo(i64 0)
-  unreachable
-
-out_of_bounds2:
-  call void @foo(i64 %val1)
-  unreachable
-}
-
-declare void @foo(i64)
-