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: unassignedb...@nondot.org
Reporter: rmor...@synopsys.com
CC: llvm-bugs@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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