http://llvm.org/bugs/show_bug.cgi?id=17616
Bug ID: 17616
Summary: pointer arithmetic simplification
Product: libraries
Version: trunk
Hardware: PC
OS: Linux
Status: NEW
Severity: normal
Priority: P
Component: Scalar Optimizations
Assignee: [email protected]
Reporter: [email protected]
CC: [email protected]
Classification: Unclassified
int* f(int*b,int*e){
return b+(e-b);
}
generates this code on x86_64:
subq %rdi, %rsi
andq $-4, %rsi
addq %rdi, %rsi
movq %rsi, %rax
or in llvm terms:
%sub.ptr.lhs.cast = ptrtoint i32* %e to i64
%sub.ptr.rhs.cast = ptrtoint i32* %b to i64
%sub.ptr.sub = sub i64 %sub.ptr.lhs.cast, %sub.ptr.rhs.cast
%sub.ptr.div = ashr exact i64 %sub.ptr.sub, 2
%add.ptr = getelementptr inbounds i32* %b, i64 %sub.ptr.div
ret i32* %add.ptr
I'm sure you can guess what optimization I'd like to see...
Note that this happens in real code, for instance as a way to cast away
constness if e was const int*.
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