[PATCH][RTL ifcvt] PR rtl-optimization/71594: ICE in noce_emit_cmove due to mismatched source modes

[Kyrill Tkachov <kyrylo dot tkachov at foss dot arm dot com>]

Hi all,

In this PR we get an ICE when trying to emit a conditional move through noce_convert_multiple_sets.
The comment in the patch explains the situation but we get a two-instruction sequence like:
(insn 20 19 21 3 (set (reg:SI 89 [ _5 ])
        (reg:SI 88 [ _4 ])) wice.c:8 82 {*movsi_internal}
     (nil))
(insn 21 20 25 3 (set (reg:HI 90 [ a_lsm.10 ])
        (subreg:HI (reg:SI 89 [ _5 ]) 0)) wice.c:8 84 {*movhi_internal}
     (nil))

The first instruction feeds the second, but the second takes the lowpart subreg of the first destination.
This leads to the noce_emit_cmove call taking as arguments the first SImode destination and the second HImode
destination.  This causes an assertion failure deeper down the line.

The solution in this patch is catch this case and wrap the first destination in a lowpart subreg so that the two
operands of the cmove have the same mode.

Bootstrapped and tested on aarch64-none-linux-gnu, arm-none-linux-gnueabihf and x86_64-unknown-linux-gnu.

Ok for trunk?

Thanks,
Kyrill

2016-06-24  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    PR rtl-optimization/71594
    * ifcvt.c (noce_convert_multiple_sets): Wrap new_val or old_val
    into subregs of appropriate mode before trying to emit a conditional
    move.

2016-06-24  Kyrylo Tkachov  <kyrylo.tkachov@arm.com>

    PR rtl-optimization/71594
    * gcc.dg/torture/pr71594.c: New test.

diff --git a/gcc/ifcvt.c b/gcc/ifcvt.c
index fd2951673fb6bd6d9e5d52cdb88765434a603fb6..3617de674561019a8dd155f13391c6eb3cbac1e4 100644
--- a/gcc/ifcvt.c
+++ b/gcc/ifcvt.c
@@ -3228,6 +3228,33 @@ noce_convert_multiple_sets (struct noce_if_info *if_info)
       if (if_info->then_else_reversed)
 	std::swap (old_val, new_val);
 
+
+      /* We allow simple lowpart register subreg SET sources in
+	 bb_ok_for_noce_convert_multiple_sets.  Be careful when processing
+	 sequences like:
+	 (set (reg:SI r1) (reg:SI r2))
+	 (set (reg:HI r3) (subreg:HI (r1)))
+	 For the second insn new_val or old_val (r1 in this example) will be
+	 taken from the temporaries and have the wider mode which will not
+	 match with the mode of the other source of the conditional move, so
+	 we'll end up trying to emit r4:HI = cond ? (r1:SI) : (r3:HI).
+	 Wrap the two cmove operands into subregs if appropriate to prevent
+	 that.  */
+      if (GET_MODE (new_val) != GET_MODE (temp))
+	{
+	  machine_mode src_mode = GET_MODE (new_val);
+	  machine_mode dst_mode = GET_MODE (temp);
+	  gcc_assert (GET_MODE_SIZE (src_mode) > GET_MODE_SIZE (dst_mode));
+	  new_val = lowpart_subreg (dst_mode, new_val, src_mode);
+	}
+      if (GET_MODE (old_val) != GET_MODE (temp))
+	{
+	  machine_mode src_mode = GET_MODE (old_val);
+	  machine_mode dst_mode = GET_MODE (temp);
+	  gcc_assert (GET_MODE_SIZE (src_mode) > GET_MODE_SIZE (dst_mode));
+	  old_val = lowpart_subreg (dst_mode, old_val, src_mode);
+	}
+
       /* Actually emit the conditional move.  */
       rtx temp_dest = noce_emit_cmove (if_info, temp, cond_code,
 				       x, y, new_val, old_val);
diff --git a/gcc/testsuite/gcc.dg/torture/pr71594.c b/gcc/testsuite/gcc.dg/torture/pr71594.c
new file mode 100644
index 0000000000000000000000000000000000000000..468a9f6891c92ff76520af0eee242f08b01ae0cf
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/torture/pr71594.c
@@ -0,0 +1,15 @@
+/* { dg-do compile } */
+/* { dg-options "--param max-rtl-if-conversion-insns=2" } */
+
+unsigned short a;
+int b, c;
+int *d;
+void fn1() {
+  *d = 24;
+  for (; *d <= 65;) {
+    unsigned short *e = &a;
+    b = (a &= 0 <= 0) < (c ?: (*e %= *d));
+    for (; *d <= 83;)
+      ;
+  }
+}