https://bugs.llvm.org/show_bug.cgi?id=34228
Bug ID: 34228
Summary: Optimizer doesn't eliminate select with two equivalent
operands
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: enhancement
Priority: P
Component: Scalar Optimizations
Assignee: unassignedb...@nondot.org
Reporter: m...@manueljacob.de
CC: llvm-bugs@lists.llvm.org
The following transformation doesn't happen:
%shared = add i64 %x, %y
%r1 = select i1 %c1, i64 %shared, i64 %x
%r2 = select i1 %c2, i64 %shared, i64 %y
%tmp = select i1 %c2, i64 %x, i64 0
%r2_eq1 = add i64 %tmp, %y
%r2_eq2 = select i1 %zzz, i64 %r2, i64 %r2_eq1
%r = add i64 %r1, %r2_eq2
=>
%shared = add i64 %x, %y
%r1 = select i1 %c1, i64 %shared, i64 %x
%r2 = select i1 %c2, i64 %shared, i64 %y
%r = add i64 %r1, %r2
http://rise4fun.com/Alive/IOS5
InstCombine can do the following canonicalization (see comment in
`getSelectFoldableOperands()`):
%shared = add i64 %x, %y
%r2 = select i1 %c2, i64 %shared, i64 %y
=>
%tmp = select i1 %c2, i64 %x, i64 0
%r2 = add i64 %tmp, %y
But in this case the canonicalization doesn't happen, because %shared has two
uses. Note that %r2_eq1 actually is the canonical form of %r2. Further
canonicalization of either %r2 or %r2_eq1 won't happen. This is why the
optimizer doesn't see that %r2_eq2 selects between two equivalent operands (and
is therefore equal to %r2).
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