i386: Fix handling of SUBREGs in divv2sf3 [PR103842]

register_operand predicate allows not just REGs, but also SUBREGs of REGs,
and for the latter lowpart_subreg might FAIL when trying to create paradoxical
SUBREG in some cases.  For the input operand fixed by force_reg on it first,
for the output operand handled by always dividing into a fresh V4SFmode temporary
and emit_move_insn into the destination afterwards, that is also beneficial for
combine.

2021-12-28  Jakub Jelinek  <jakub@redhat.com>

	PR target/103842
	* config/i386/mmx.md (divv2sf3): Use force_reg on op1.  Always perform
	divv4sf3 into a pseudo and emit_move_insn into operands[0].

	* g++.dg/opt/pr103842.C: New test.

2 files changed, 37 insertions, 4 deletions

diff --git a/gcc/config/i386/mmx.md b/gcc/config/i386/mmx.md
index 5a57556..e394cba 100644
--- a/gcc/config/i386/mmx.md
+++ b/gcc/config/i386/mmx.md
@@ -529,17 +529,19 @@
 		  (match_operand:V2SF 2 "register_operand")))]
   "TARGET_MMX_WITH_SSE"
 {
-  rtx op0 = lowpart_subreg (V4SFmode, operands[0],
-			    GET_MODE (operands[0]));
-  rtx op1 = lowpart_subreg (V4SFmode, operands[1],
-			    GET_MODE (operands[1]));
+  rtx op1 = lowpart_subreg (V4SFmode, force_reg (V2SFmode, operands[1]),
+			    V2SFmode);
   rtx op2 = gen_rtx_VEC_CONCAT (V4SFmode, operands[2],
 				force_reg (V2SFmode, CONST1_RTX (V2SFmode)));
   rtx tmp = gen_reg_rtx (V4SFmode);
 
   emit_insn (gen_rtx_SET (tmp, op2));
 
+  rtx op0 = gen_reg_rtx (V4SFmode);
+
   emit_insn (gen_divv4sf3 (op0, op1, tmp));
+
+  emit_move_insn (operands[0], lowpart_subreg (V2SFmode, op0, V4SFmode));
   DONE;
 })
 
diff --git a/gcc/testsuite/g++.dg/opt/pr103842.C b/gcc/testsuite/g++.dg/opt/pr103842.C
new file mode 100644
index 0000000..4fbafe3
--- /dev/null
+++ b/gcc/testsuite/g++.dg/opt/pr103842.C
@@ -0,0 +1,31 @@
+// PR target/103842
+// { dg-do compile }
+// { dg-options "-O3 -std=c++14" }
+
+void foo (float *);
+struct M {
+  float x[3][3];
+  float *operator[](int i) { return x[i]; }
+  M();
+  M(float f, float g) {
+    x[1][0] = x[1][1] = x[1][2] = f;
+    x[2][0] = g;
+  }
+  void bar();
+  M baz() {
+    M s(x[1][2] - x[1][2], x[1][1] - x[1][1]);
+    float r = s[2][0];
+    if (r)
+      for (int i = 0; i < 3; ++i)
+	for (int j = 0; j < 3; ++j)
+	  s[i][j] /= r;
+    for (int i = 0;;) {
+      float *t = s[i];
+      foo(t);
+    }
+  }
+};
+void qux() {
+  M m, i = m.baz(), j = i;
+  j.bar();
+}