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From: joeytsai@yahoo.com To: gcc-gnats@gcc.gnu.org Subject: c/8825: optimization causes incorrect math Date: Thu, 05 Dec 2002 10:36:00 -0000 [thread overview] Message-ID: <20021205183537.18136.qmail@sources.redhat.com> (raw) >Number: 8825 >Category: c >Synopsis: optimization causes incorrect math >Confidential: no >Severity: serious >Priority: medium >Responsible: unassigned >State: open >Class: sw-bug >Submitter-Id: net >Arrival-Date: Thu Dec 05 10:36:02 PST 2002 >Closed-Date: >Last-Modified: >Originator: Joey Tsai >Release: gcc version 2.95.4 20011002 (Debian prerelease) >Organization: >Environment: Debian Unstable >Description: /* It appears that using optimization causes some weird math to happen. In this example below, there is a point which is being updated by another structure which has more precision. But when using optimization, an unexpected result occurs, specifically 112+0=111. */ #include <stdio.h> #include <math.h> struct _move { double x, y; double dx, dy; } move; struct _point { int x, y; } point; void setup_move( struct _move * m, int angle, int x, int y ) { double d; d = (double) angle * (double) M_PI / (double) 180.0; m->dx = cos(d) * (double) -10.0; m->dy = sin(d) * (double) -10.0; m->x = (double) x; m->y = (double) y; } int main( int argc, char * argv[] ) { point.x = 112; point.y = 400; setup_move( &move, 90, point.x, point.y ); move.x += move.dx; move.y += move.dy; point.x = (int) move.x; point.y = (int) move.y; printf( "point x=%d, y=%d\n", point.x, point.y ); return 0; } >How-To-Repeat: This is incorrect: $ gcc -O1 -o go bob.c -lm $ ./go point x=111, y=390 This is correct: $ gcc -o go bob.c -lm $ ./go point x=112, y=390 >Fix: Please let me know if there's a work-around, I'd like to turn on optimization. >Release-Note: >Audit-Trail: >Unformatted:
next reply other threads:[~2002-12-05 18:36 UTC|newest] Thread overview: 2+ messages / expand[flat|nested] mbox.gz Atom feed top 2002-12-05 10:36 joeytsai [this message] 2002-12-05 11:19 bangerth
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