[LLVMdev] Dependence Analysis [was: Flow-Sensitive AA]
clattner at apple.com
Fri Aug 22 17:54:24 PDT 2008
On Aug 22, 2008, at 3:05 PM, Vikram S. Adve wrote:
> On Aug 22, 2008, at 4:49 PM, John Regehr wrote:
>> Has anyone quantified the optimizations afforded by undefined signed
>> overflow? I'd expect that the benefits are minimal for most codes.
> In most cases, I agree. But for codes that depend heavily on
> dependence analysis, I would think that being conservative with index
> expressions would really kill any disambiguation capability and make
> many loop optimizations and other dependence-based optimizations much
> weaker. For example, static scheduling of array intensive loops seems
> vulnerable to this.
There are many 'instcombine' level optimizations that become safe when
overflow can't happen, e.g. x*2/2 -> x.
However, by far the most important case is loop induction variables
for realistic code.
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