Compile the following with -O2 -funroll-loops and you get bad codegen. An uninitilaized value is used as "pos" in computing the store address for mr[pos]=1 at the top of the function. The first dump where it's wrong seems to be "nrv". If you unroll by hand it comes out correct.
extern int delta[8]; extern unsigned char board[421]; void ping_recurse(int pos, int mr[400], int color) { int k; mr[pos] = 1; for (k = 0; k < 4; k++) { int apos = pos + delta[k]; if (mr[apos] == 1 || board[apos] == color) ping_recurse(apos, mr, color); } } Beginning of codegen on darwin8: _ping_recurse: mflr r0 bcl 20,31,"L00000000001$pb" "L00000000001$pb": stmw r28,-16(r1) mr r29,r4 mflr r31 mr r28,r5 stw r0,8(r1) stwu r1,-80(r1) L25: addis r11,r31,ha16(L_delta$non_lazy_ptr-"L00000000001$pb") slwi r2,r30,2 <<<< R30 uninitiailized -- Summary: Quiet bad codegen (unrolling + tail call interaction) Product: gcc Version: 4.0.0 Status: UNCONFIRMED Keywords: wrong-code Severity: normal Priority: P2 Component: tree-optimization AssignedTo: unassigned at gcc dot gnu dot org ReportedBy: dalej at gcc dot gnu dot org CC: gcc-bugs at gcc dot gnu dot org GCC build triplet: powerpc-apple-darwin8.0.0 GCC host triplet: powerpc-apple-darwin8.0.0 GCC target triplet: powerpc-apple-darwin8.0.0 http://gcc.gnu.org/bugzilla/show_bug.cgi?id=21562