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[Bug tree-optimization/77859] New: Ofast needed in presence of conditional code
- From: "vincenzo.innocente at cern dot ch" <gcc-bugzilla at gcc dot gnu dot org>
- To: gcc-bugs at gcc dot gnu dot org
- Date: Wed, 05 Oct 2016 08:29:29 +0000
- Subject: [Bug tree-optimization/77859] New: Ofast needed in presence of conditional code
- Auto-submitted: auto-generated
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77859
Bug ID: 77859
Summary: Ofast needed in presence of conditional code
Product: gcc
Version: 7.0
Status: UNCONFIRMED
Severity: normal
Priority: P3
Component: tree-optimization
Assignee: unassigned at gcc dot gnu.org
Reporter: vincenzo.innocente at cern dot ch
Target Milestone: ---
It looks to me that to vectorize this code "relaxed floating point math" is not
a requirement
currently gcc version 7.0.0 20161004 (experimental) [trunk revision 240754]
(GCC)
requires Ofast and not O3 to vectorize it
#include <cmath>
float x[1024];
float y[1024];
void needOfast() {
for (int i=0; i<1024; ++i) {
constexpr float pi4 = M_PI/4.;
constexpr float pi2 = M_PI/2.;
auto g1 = x[i] > pi4;
auto xx = x[i];
xx = g1 ? xx-pi2 : xx;
auto g2 = xx > pi4;
xx = g2 ? xx-pi2 : xx;
y[i] = xx;
}
}
in case anyone wonder this alternative formulation needs Ofast as well
void needOfastAsWell() {
for (int i=0; i<1024; ++i) {
constexpr float pi = M_PI;
constexpr float pi4 = M_PI/4.;
constexpr float pi34 = 3.*M_PI/4.;
constexpr float pi2 = M_PI/2.;
auto g1 = x[i] > pi4;
auto xx = x[i];
xx = g1 ? xx-pi2 : xx;
auto g2 = x[i] > pi34;
xx = g2 ? x[i]-pi : xx;
y[i] = xx;
}
}