[Bug rtl-optimization/99221] New: copyprop_hardreg_forward_1 deletes insn by mistake

[Bug rtl-optimization/99221] New: copyprop_hardreg_forward_1 deletes insn by mistake

[stefansf at linux dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99221

            Bug ID: 99221
           Summary: copyprop_hardreg_forward_1 deletes insn by mistake
           Product: gcc
           Version: 11.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: rtl-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: stefansf at linux dot ibm.com
  Target Milestone: ---

Created attachment 50242
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50242&action=edit
a-foo.i.301r.jump2

Consider the following reduced example:

int g = 9, h, i, b;
char j, k;
long m;
char *a = &j;
char n() { return 0; }
void o(short o) {}
short p(short o, short q) { return q; }
long r(long s) { return s; }
long t(long s) { return 0; }
long u(long s) { return 0; }
short v() { return 0; }
void x(int **s, unsigned q) {
   short c = 0, d = 0;
   int e = 4;
   if (g) {
     int f = 4;
     q = 1;
     for (; q <= 4; q++) {
       m = q;
       d = i = u(d);
       if (0 == r(1)) {
         if (u(0)) {
           h = t(q) != **s;
           for (; e; e = f &= 0 <= 0)
             ;
         }
       } else {
         b = c;
         c = v();
         o(q);
         k = n();
         j = p(k || q, q);
       }
     }
   }
}
int main() {
   x(0, 0);
   printf("%d\n", j);
   return 0;
}

Command line: gcc -march=arch11 -w -Og foo.i && ./a.out
Expected output: 4
Actual output: 0

On IBM Z we have prior pass cprop_hardreg the following insns of interest:

(insn 24 23 25 4 (set (reg:DI 24 %f8 [orig:61 _2 ] [61])
        (reg/v:DI 12 %r12 [orig:85 q+-4 ] [85])) "./foo.i":19:10 1462
{*movdi_64}
     (nil))
...
(insn 80 79 81 10 (set (reg:HI 24 %f8 [orig:74 _15 ] [74])
        (reg:HI 12 %r12 [orig:85 q+2 ] [85])) "./foo.i":30:10 1474 {*movhi}
     (nil))
...
(insn 155 96 97 15 (set (reg:HI 1 %r1 [orig:74 _15 ] [74])
        (reg:HI 24 %f8 [orig:74 _15 ] [74])) "./foo.i":32:14 1474 {*movhi}
     (nil))

Register f8 is set to a 64-bit value in insn 24 and to a 16-bit value in insn
80, respectively, while using the same source register r12. During
copyprop_hardreg_forward_1 it is then wrongly detected that insn 80 is a noop
set and is subsequently removed. Due to different alignments of different modes
in FPRs we have that in insn 155 the wrong part of register f8 is then accessed
which results in constant value zero.

In order to decide whether an insn is a noop set or not I gave it a try by
additionally testing whether the prior set and the current are done in
compatible modes by asking the backend:

diff --git a/gcc/regcprop.c b/gcc/regcprop.c
index e1342f56bd1..02753a12510 100644
--- a/gcc/regcprop.c
+++ b/gcc/regcprop.c
@@ -474,7 +474,8 @@ find_oldest_value_reg (enum reg_class cl, rtx reg, struct
value_data *vd)
        (set (...) (reg:DI r9))
      Replacing r9 with r11 is invalid.  */
   if (mode != vd->e[regno].mode
-      && REG_NREGS (reg) > hard_regno_nregs (regno, vd->e[regno].mode))
+      && (REG_NREGS (reg) > hard_regno_nregs (regno, vd->e[regno].mode)
+         || !REG_CAN_CHANGE_MODE_P (regno, mode, vd->e[regno].mode)))
     return NULL_RTX;

   for (i = vd->e[regno].oldest_regno; i != regno; i = vd->e[i].next_regno)

Any thoughts about this fix?

[Bug rtl-optimization/99221] copyprop_hardreg_forward_1 deletes insn by mistake

[stefansf at linux dot ibm.com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=99221

--- Comment #1 from Stefan Schulze Frielinghaus <stefansf at linux dot ibm.com> ---
Created attachment 50243
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=50243&action=edit
a-foo.i.307r.cprop_hardreg