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: [email protected]
ReportedBy: [email protected]
CC: [email protected], [email protected]
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?
