[Bug tree-optimization/40384] [4.5 regression] Revision 148277 failed gcc.dg/tree-ssa/prefetch-5.c

["hjl dot tools at gmail dot com" <gcc-bugzilla@gcc.gnu.org>]

------- Comment #3 from hjl dot tools at gmail dot com  2009-06-10 20:14 -------
(In reply to comment #2)
> The new cost model added in revision 148277 supresses prefetching in a loop
> when it is unlikely to be profitable. One such non-profitable case is a loop
> with an unknown trip count and a high prefeching cost. The prefetching cost is
> modeled by the ratio between the prefetch count and the total instruction count
> in a loop. When this ratio is greater than 1/10 in a loop with an unknown trip
> count, prefetching is considered non-profitable and no prefetches are issued.
> The first two loops in this test case fall under this category, where the trip
> count is not known and the prefetch-to-insn ratio is 1/5. Possible fixes are:
> 
> 1. Add more computation to the loop bodies in the test case to reduce the
> prefetch-to-insn ratios and hence make the loops profitable from the cost
> model's point of view.
> 2. Use a commad line option to set the maximum profitable prefetch-to-insn
> ratio to 1/5. The syntax is --param min-insn-to-prefetch-ratio=5
> 3. Change the expected output such that it does not expect prefetches in this
> case.
> 

I prefer #2 since this is a prefetch test.


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hjl dot tools at gmail dot com changed:

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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40384