http://llvm.org/bugs/show_bug.cgi?id=21274
Bug ID: 21274
Summary: InstCombine: ((1 << A) >>u B) --> (1 << (A-B))
incorrect if A-B > bitwidth
Product: libraries
Version: trunk
Hardware: All
OS: All
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
simplifyValueKnownNonZero() in InstCombineMulDivRem.cpp simplifies the RHS of
div/rem instructions.
However, the first transformation it does on RHS doesn't seem correct:
((1 << A) >>u B) --> (1 << (A-B))
Pre: isPowerOf2(%Power)
%shl = shl %Power, %A
%Y = lshr %shl, %B
%r = udiv %X, %Y
=>
%sub = sub %A, %B
%Y = shl %Power, %sub
%r = udiv %X, %Y
Alive says the following:
ERROR: Domain of definedness of Target is smaller than Source's for i4 %Y
Example:
%Power i4 = 0x4 (4)
%A i4 = 0x0 (0)
%B i4 = 0x1 (1)
%X i4 = 0x0 (0)
%shl i4 = 0x4 (4)
%sub i4 = 0xF (15, -1)
Source value: 0x2 (2)
Target value: undef
Can't see a fix other than removing this transformation altogether.
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