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From: "spatel at rotateright dot com" <gcc-bugzilla@gcc.gnu.org> To: gcc-bugs@gcc.gnu.org Subject: [Bug target/62191] New: extra shift generated for vector integer division by constant 2 Date: Tue, 19 Aug 2014 18:24:00 -0000 [thread overview] Message-ID: <bug-62191-4@http.gcc.gnu.org/bugzilla/> (raw) https://gcc.gnu.org/bugzilla/show_bug.cgi?id=62191 Bug ID: 62191 Summary: extra shift generated for vector integer division by constant 2 Product: gcc Version: 4.9.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: target Assignee: unassigned at gcc dot gnu.org Reporter: spatel at rotateright dot com Using gcc 4.9: $ cat sdiv.c typedef int vecint __attribute__((vector_size(16))); vecint f(vecint x) { return x/2; } $ gcc -O2 sdiv.c -S -o - ... movdqa %xmm0, %xmm1 psrad $31, %xmm1 <--- splat the sign bit psrld $31, %xmm1 <--- then shift sign bit down to LSB paddd %xmm1, %xmm0 <--- add sign bit to quotient psrad $1, %xmm0 <--- div via alg shift right ret -------------------------------------------------------------- I don't think the first shift right algebraic is necessary. We splat the sign bit and then shift that right logically, so the upper bits are all zero'd anyway. This is a special case for signed integer division by 2. You need that first 'psrad' for any other power of 2 because the subsequent logical shift would not also be a shift of 31.
next reply other threads:[~2014-08-19 18:24 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2014-08-19 18:24 spatel at rotateright dot com [this message] 2020-09-03 20:50 ` [Bug target/62191] " gabravier at gmail dot com
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