Because for integer shift instructions the shift count is truncated. We ensure that we only use integer shift instructions by emitting a shift with a mask. This only matches integer shift instructions in the md file.
-----Original Message----- From: Richard Kenner [mailto:ken...@vlsi1.ultra.nyu.edu] Sent: Tuesday, August 8, 2017 12:52 PM To: Michael Collison <michael.colli...@arm.com> Cc: gcc-patches@gcc.gnu.org; nd <n...@arm.com>; pins...@gmail.com Subject: RE: [PATCH] [Aarch64] Optimize subtract in shift counts > This case is covered by Wilco's previous reply: > > https://gcc.gnu.org/ml/gcc-patches/2017-08/msg00575.html Which I don't understand: > No it's perfectly safe - it becomes an integer-only shift after the > split since it keeps the masking as part of the pattern. Let say we have your first example: long f1(long x, int i) { return x >> (64 - i); } If "i" is -2, this should be a shift of 66 (which is indeed, technically undefined), but becomes a shift of 62. What am I missing?
