[llvm-bugs] [Bug 28439] New: ScalarEvolution creates redundant select from phi

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=28439

            Bug ID: 28439
           Summary: ScalarEvolution creates redundant select from phi
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Global Analyses
          Assignee: unassignedbugs at nondot.org
          Reporter: rmoryle at synopsys.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

Created attachment 16699
  --> https://llvm.org/bugs/attachment.cgi?id=16699&action=edit
C example

After http://reviews.llvm.org/D13378 SE can create SCEV expressions from
select-like phi nodes. But when such expression is expanded, instead of reusing
original phi node, SEExpander creates new select+icmp. This leads to code
duplication.

Simple example is attached. Use 'clang -O2 -S -emit-llvm test.c' to repoduce.

Output with select-like-phi recognition:

if.end:                                           ; preds = %if.else, %if.then
  %N.0 = phi i32 [ %A, %if.then ], [ %B, %if.else ]
  %cmp17 = icmp sgt i32 %N.0, 0
  br i1 %cmp17, label %for.body.preheader, label %for.end

for.body.preheader:                               ; preds = %if.end
  %0 = icmp ult i32 %B, %A          <<<<< This two instruction 
  %1 = select i1 %0, i32 %B, i32 %A <<<<< can be replaced with %N.0 phi
  %2 = add i32 %1, -1
  %3 = zext i32 %2 to i64
  %4 = add nuw nsw i64 %3, 1
  %scevgep = getelementptr i32, i32* %P, i64 %4
  br label %for.end

for.end:
  %P.addr.0.lcssa = phi i32* [ %P, %if.end ], [ %scevgep, %for.body.preheader ]
  ret i32* %P.addr.0.lcssa

Code without select-like-phi recognition:

if.end:                                           ; preds = %if.else, %if.then
  %N.0 = phi i32 [ %A, %if.then ], [ %B, %if.else ]
  %cmp17 = icmp sgt i32 %N.0, 0
  %0 = add i32 %N.0, -1
  %1 = zext i32 %0 to i64
  %2 = add nuw nsw i64 %1, 1
  %scevgep = getelementptr i32, i32* %P, i64 %2
  %P.addr.0.lcssa = select i1 %cmp17, i32* %scevgep, i32* %P
  ret i32* %P.addr.0.lcssa

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