http://llvm.org/bugs/show_bug.cgi?id=21255
Bug ID: 21255
Summary: InstCombine: '(x lshr C1) udiv C2 --> x udiv (C2 <<
C1)' incorrect if C2<<C1 overflows
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
The following transformation in InstCombiner::visitUDiv() is incorrect if
C2<<C1 overflows (and btw correct otherwise):
; (x lshr C1) udiv C2 --> x udiv (C2 << C1)
%Op0 = lshr %X, C1
%r = udiv %Op0, C2
=>
%r = udiv %X, (C2 << C1)
Done: 1
ERROR: Domain of definedness of Target is smaller than Source's for i2 %r
Example:
%X i2 = 0x0 (0)
C1 i2 = 0x1 (1)
C2 i2 = 0x2 (2, -2)
%Op0 i2 = 0x0 (0)
Source value: 0x0 (0)
Target value: undef
And bypassing the undef case:
Pre: ((C2 << C1) != 0)
%Op0 = lshr exact %X, C1
%r = udiv %Op0, C2
=>
%r = udiv %X, (C2 << C1)
ERROR: Mismatch in values of i4 %r
Example:
%X i4 = 0x2 (2)
C1 i4 = 0x1 (1)
C2 i4 = 0x9 (9, -7)
%Op0 i4 = 0x1 (1)
Source value: 0x0 (0)
Target value: 0x1 (1)
And finally we have:
Pre: WillNotOverflowUnsignedShl(C2, C1)
%Op0 = lshr %X, C1
%r = udiv %Op0, C2
=>
%r = udiv %X, (C2 << C1)
Done
Optimization is correct!
--
You are receiving this mail because:
You are on the CC list for the bug.
_______________________________________________
LLVMbugs mailing list
[email protected]
http://lists.cs.uiuc.edu/mailman/listinfo/llvmbugs
