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This "subtract/extend/add" existed for a long time and still annoying us (PR37451, part of PR61837) when converting from 32bits to 64bits, as the ctr register is used as 64bits on powerpc64, Andraw Pinski had a patch but caused some issue and reverted by Joseph S. Myers(PR37451, PR37782). Andraw: http://gcc.gnu.org/ml/gcc-patches/2008-09/msg01070.html http://gcc.gnu.org/ml/gcc-patches/2008-10/msg01321.html Joseph: https://gcc.gnu.org/legacy-ml/gcc-patches/2011-11/msg02405.html We still can do the simplification from "subtract/zero_ext/add" to "zero_ext" when loop iterations is known to be LT than MODE_MAX (only do simplify when counter+0x1 NOT overflow). Bootstrap and regression tested pass on Power8-LE. gcc/ChangeLog 2020-05-15 Xiong Hu Luo <luoxhu@linux.ibm.com> PR rtl-optimization/37451, part of PR target/61837 * loop-doloop.c (doloop_simplify_count): New function. Simplify (add -1; zero_ext; add +1) to zero_ext when not wrapping. (doloop_modify): Call doloop_simplify_count. gcc/testsuite/ChangeLog 2020-05-15 Xiong Hu Luo <luoxhu@linux.ibm.com> PR rtl-optimization/37451, part of PR target/61837 * gcc.target/powerpc/doloop-2.c: New test.
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