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*From*: Peter Steinmetz <steinmtz at us dot ibm dot com>*To*: gcc-patches at gcc dot gnu dot org*Date*: Thu, 1 Sep 2005 10:28:57 -0500*Subject*: [PATCH,4.2] Use edge probabilities to control speculation in interblockscheduler

The interblock instruction scheduler computes static probabilities for reaching the blocks of the region being scheduled. It starts at the region entry point with an assumed probability of 100% and distributes that downward to the other blocks of the region, equally dividing the probability amongst successor blocks, and assuming a small rate of escape along paths that exit the region. The scheduler uses these computed block probabilities to determine the potential usefulness of speculatively moving an instruction from one block to another. A more accurate way to distribute the probability is to make use of the edge probability data in the control flow graph. In the absence of FDO, the edge probabilities are set by static branch prediction. When FDO data is present, the actual counts are used to set the edge probabilities. Using this approach gives the interblock scheduler a more accurate view of the hot paths through the region and results in smarter decisions as to which paths should be favored for speculative motions, and which paths should be avoided. Performance tests show that the generated code using edge probabilities behaves equally well or better on PowerPC. Bootstrapped and regtested on powerpc64-unknown-linux-gnu with no failures. Ok for 4.2? .. could be considered for 4.1 since the code impact is minimal and there is potential for improved performance. Pete. (See attached file: patch.use-edge-probabilities)

**Attachment:
patch.use-edge-probabilities**

**Follow-Ups**:**Re: [PATCH,4.2] Use edge probabilities to control speculation in interblock scheduler***From:*Richard Henderson

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