http://llvm.org/bugs/show_bug.cgi?id=16891
Bug ID: 16891
Summary: Optimization bug with -Ofast
Product: clang
Version: 3.3
Hardware: Macintosh
OS: MacOS X
Status: NEW
Severity: normal
Priority: P
Component: -New Bugs
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
Created attachment 11036
--> http://llvm.org/bugs/attachment.cgi?id=11036&action=edit
Example program showing the issue.
This bug appears when compiling for iOS Simulator or iOS device using this
Clang version:
Apple LLVM version 5.0 (clang-500.1.63) (based on LLVM 3.3svn)
When compiling with -Ofast, a specific loop produces incorrect resuls. Works
with -Os. It appears that the optimizer believes a long value bitor another
long value shifted by 32 results in -1 instead of just remaining the same value
as you started with. The attached code works if both "long" are replaced with
int64_t.
Steps to Reproduce:
Compile attached code with:
clang -arch i386 -Os ~/bug.cpp -o bugopts
to get this result:
Val = 1, byte = 1
Val = 5633, byte = 22
Val = 5633, byte = 0
Val = 5633, byte = 0
Val = 5633, byte = 0
Val = 5633, byte = 0
Val = 5633, byte = 0
Val = 5633, byte = 0
Value is 5633
Use this:
clang -arch i386 -Ofast ~/bug.cpp -o bugoptfast
and you get this result:
Val = 1, byte = 1
Val = 5633, byte = 22
Val = 5633, byte = 0
Val = 5633, byte = 0
Val = -1, byte = 0
Val = -1, byte = 0
Val = -1, byte = 0
Val = -1, byte = 0
Value is 0
Also logged with Apple as issue 14739667
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