regrename: Fix -fcompare-debug issue in check_new_reg_p [PR105041]

In check_new_reg_p, the nregs of a du chain is computed by obtaining the
MODE of the first element in the chain, and then calling
hard_regno_nregs() with the MODE. But the first element of the chain can
be a DEBUG_INSN whose mode need not be the same as the rest of the
elements in the du chain. This was resulting in fcompare-debug failure
as check_new_reg_p was returning a different result with -g for the same
candidate register. We can instead obtain nregs from the du chain
itself.

2022-06-10  Surya Kumari Jangala  <[email protected]>

gcc/
	PR rtl-optimization/105041
	* regrename.c (check_new_reg_p): Use nregs value from du chain.

gcc/testsuite/
	PR rtl-optimization/105041
	* gcc.target/powerpc/pr105041.c: New test.

(cherry picked from commit 3e16b43)

Author: Surya Kumari Jangala

gcc/regrename.c

@@ -324,8 +324,7 @@ static bool
 check_new_reg_p (int reg ATTRIBUTE_UNUSED, int new_reg,
 		 class du_head *this_head, HARD_REG_SET this_unavailable)
 {
-  machine_mode mode = GET_MODE (*this_head->first->loc);
-  int nregs = hard_regno_nregs (new_reg, mode);
+  int nregs = this_head->nregs;
   int i;
   struct du_chain *tmp;
 

gcc/testsuite/gcc.target/powerpc/pr105041.c

@@ -0,0 +1,22 @@
+/* { dg-options "-mdejagnu-cpu=power4 -O2 -fcompare-debug -frename-registers" } */
+
+double m;
+int n;
+
+unsigned int
+foo (unsigned int x, int y)
+{
+  long long int a = y, b = !a;
+  int c = 0;
+
+  if (b != x)
+    while ((int) m == a)
+      {
+        c = a;
+        a = 0;
+      }
+
+  n = b = y;
+
+  return x + c;
+}