http://gcc.gnu.org/bugzilla/show_bug.cgi?id=47299
Summary: Widening multiply optimization generates bad code Product: gcc Version: 4.5.0 Status: UNCONFIRMED Keywords: wrong-code Severity: normal Priority: P3 Component: rtl-optimization AssignedTo: unassig...@gcc.gnu.org ReportedBy: uweig...@gcc.gnu.org CC: ber...@codesourcery.com, rguent...@suse.de Building the following test case with current mainline on i386: unsigned short test (unsigned char val) __attribute__ ((noinline)); unsigned short test (unsigned char val) { return val * 255; } int main(int argc, char**argv) { printf ("test(val=40) = %x\n", test(0x40)); return 0; } We get the following (correct) output with -O0: test(val=40) = 3fc0 and the following incorrect output with -O2: test(val=40) = ffc0 The problem appears to be related to this piece of code in expand_expr_real2, case WIDEN_MULT_EXPR: expand_operands (treeop0, treeop1, NULL_RTX, &op0, &op1, EXPAND_NORMAL); temp = expand_widening_mult (mode, op0, op1, target, unsignedp, this_optab); expand_operands will expand the constant 255 into QImode and return a (const_int -1) for op1. Passing this constant into expand_widening_mult then apparently generates a simple negation operation in HImode instead (via expand_const_mult) ... It seems this code came in here: http://gcc.gnu.org/ml/gcc-patches/2010-04/msg01327.html Any suggestions how this ought to be handled?