[llvm-bugs] [Bug 25578] New: IndVarSimplify breaks LCSSA form, while saying it's preserved

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=25578

            Bug ID: 25578
           Summary: IndVarSimplify breaks LCSSA form, while saying it's
                    preserved
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Loop Optimizer
          Assignee: unassignedbugs at nondot.org
          Reporter: michael.v.zolotukhin at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Applied on the following test case, IndVarSimplify introduces a definition
without corresponding LCSSA phi nodes, thus breaking LCSSA:

--------------------------------------------
; RUN: opt < %s -indvars -S | FileCheck %s
target datalayout = "e-m:o-i64:64-f80:128-n8:16:32:64-S128"

; CHECK-LABEL: @foo
define void @foo() {
entry:
  br label %L1_header

L1_header:
  br label %L2_header

; CHECK: L2_header:
; CHECK: %[[INDVAR:.*]] = phi i64
L2_header:
  %i = phi i32 [ 0, %L1_header ], [ %i_next, %L2_latch ]
  %i_prom = sext i32 %i to i64
  br label %L3_header

; CHECK: L3_header:
; CHECK: %[[TRUNC:.*]] = trunc i64 %[[INDVAR]] to i32
L3_header:
  br i1 undef, label %L3_latch, label %L2_exiting_1

L3_latch:
  br i1 undef, label %L3_header, label %L2_exiting_2

; CHECK: L2_exiting_1:
; CHECK: %{{.*}} = phi i32 [ %[[TRUNC]], %L3_header ]
L2_exiting_1:
  br i1 undef, label %L2_latch, label %L1_latch

; CHECK: L2_exiting_2:
; CHECK: %{{.*}} = phi i32 [ %[[TRUNC]], %L3_latch ]
L2_exiting_2:
  br i1 undef, label %L2_latch, label %L1_latch

L2_latch:
  %i_next = add nsw i32 %i, 1
  br label %L2_header

L1_latch:
  %i_lcssa = phi i32 [ %i, %L2_exiting_1 ], [ %i, %L2_exiting_2 ]
  br i1 undef, label %exit, label %L1_header

exit:
  ret void
}
--------------------------------------------

When it promotes an i32 induction variable to i64, it has to insert trunc
instruction before uses of the original i32 variable. In this case such use is
a phi-node(%i_lcssa in L1_latch), so it finds the nearest block that dominates
all corresponding incoming blocks (L2_exiting_1 and L2_exiting_2), and inserts
"%0 = trunc ..." there (into L3_header). However, this insertion point happens
to be in a loop (L3), so this definition is inside a loop, while its uses are
outside the loop. IndVarSimplify doesn't insert corresponding phi-nodes, and
thus breaks LCSSA.

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