gimple-fold: Fix up __builtin_clear_padding lowering [PR115527]

The builtin-clear-padding-6.c testcase fails as clear_padding_type
doesn't correctly recompute the buf->size and buf->off members after
expanding clearing of an array using a runtime loop.
buf->size should be in that case the offset after which it should continue
with next members or padding before them modulo UNITS_PER_WORD and
buf->off that offset minus buf->size.  That is what the code was doing,
but with off being the start of the loop cleared array, not its end.
So, the last hunk in gimple-fold.cc fixes that.
When adding the testcase, I've noticed that the
c-c++-common/torture/builtin-clear-padding-* tests, although clearly
written as runtime tests to test the builtins at runtime, didn't have
{ dg-do run } directive and were just compile tests because of that.
When adding that to the tests, builtin-clear-padding-1.c was already
failing without that clear_padding_type hunk too, but
builtin-clear-padding-5.c was still failing even after the change.
That is due to a bug in clear_padding_flush which the patch fixes as
well - when clear_padding_flush is called with full=true (that happens
at the end of the whole __builtin_clear_padding or on those array
padding clears done by a runtime loop), it wants to flush all the pending
padding clearings rather than just some.  If it is at the end of the whole
object, it decreases wordsize when needed to make sure the code never writes
including RMW cycles to something outside of the object:
      if ((unsigned HOST_WIDE_INT) (buf->off + i + wordsize)
          > (unsigned HOST_WIDE_INT) buf->sz)
        {
          gcc_assert (wordsize > 1);
          wordsize /= 2;
          i -= wordsize;
          continue;
        }
but if it is full==true flush in the middle, this doesn't happen, but we
still process just the buffer bytes before the current end.  If that end
is not on a wordsize boundary, e.g. on the builtin-clear-padding-5.c test
the last chunk is 2 bytes, '\0', '\xff', i is 16 and end is 18,
nonzero_last might be equal to the end - i, i.e. 2 here, but still all_ones
might be true, so in some spots we just didn't emit any clearing in that
last chunk.

2024-07-17  Jakub Jelinek  <jakub@redhat.com>

	PR middle-end/115527
	* gimple-fold.cc (clear_padding_flush): Introduce endsize
	variable and use it instead of wordsize when comparing it against
	nonzero_last.
	(clear_padding_type): Increment off by sz.

	* c-c++-common/torture/builtin-clear-padding-1.c: Add dg-do run
	directive.
	* c-c++-common/torture/builtin-clear-padding-2.c: Likewise.
	* c-c++-common/torture/builtin-clear-padding-3.c: Likewise.
	* c-c++-common/torture/builtin-clear-padding-4.c: Likewise.
	* c-c++-common/torture/builtin-clear-padding-5.c: Likewise.
	* c-c++-common/torture/builtin-clear-padding-6.c: New test.

(cherry picked from commit 8b5919bae11754f4b65a17e63663d3143f9615ac)

gcc/gimple-fold.cc

@@ -4242,7 +4242,8 @@ clear_padding_flush (clear_padding_struct *buf, bool full)
 	  i -= wordsize;
 	  continue;
 	}
-      for (size_t j = i; j < i + wordsize && j < end; j++)
+      size_t endsize = end - i > wordsize ? wordsize : end - i;
+      for (size_t j = i; j < i + endsize; j++)
 	{
 	  if (buf->buf[j])
 	    {
@@ -4271,12 +4272,12 @@ clear_padding_flush (clear_padding_struct *buf, bool full)
       if (padding_bytes)
 	{
 	  if (nonzero_first == 0
-	      && nonzero_last == wordsize
+	      && nonzero_last == endsize
 	      && all_ones)
 	    {
 	      /* All bits are padding and we had some padding
 		 before too.  Just extend it.  */
-	      padding_bytes += wordsize;
+	      padding_bytes += endsize;
 	      continue;
 	    }
 	  if (all_ones && nonzero_first == 0)
@@ -4316,7 +4317,7 @@ clear_padding_flush (clear_padding_struct *buf, bool full)
       if (nonzero_first == wordsize)
 	/* All bits in a word are 0, there are no padding bits.  */
 	continue;
-      if (all_ones && nonzero_last == wordsize)
+      if (all_ones && nonzero_last == endsize)
 	{
 	  /* All bits between nonzero_first and end of word are padding
 	     bits, start counting padding_bytes.  */
@@ -4358,7 +4359,7 @@ clear_padding_flush (clear_padding_struct *buf, bool full)
 		  j = k;
 		}
 	    }
-	  if (nonzero_last == wordsize)
+	  if (nonzero_last == endsize)
 	    padding_bytes = nonzero_last - zero_last;
 	  continue;
 	}
@@ -4832,6 +4833,7 @@ clear_padding_type (clear_padding_struct *buf, tree type,
 	  buf->off = 0;
 	  buf->size = 0;
 	  clear_padding_emit_loop (buf, elttype, end, for_auto_init);
+	  off += sz;
 	  buf->base = base;
 	  buf->sz = prev_sz;
 	  buf->align = prev_align;

gcc/testsuite/c-c++-common/torture/builtin-clear-padding-1.c

@@ -1,4 +1,5 @@
 /* PR libstdc++/88101 */
+/* { dg-do run } */
 
 int i1, i2;
 long double l1, l2;

gcc/testsuite/c-c++-common/torture/builtin-clear-padding-2.c

@@ -1,4 +1,5 @@
 /* PR libstdc++/88101 */
+/* { dg-do run } */
 
 typedef int T __attribute__((aligned (16384)));
 struct S { char a; short b; long double c; T d; T e; long long f; };

gcc/testsuite/c-c++-common/torture/builtin-clear-padding-3.c

@@ -1,4 +1,5 @@
 /* PR libstdc++/88101 */
+/* { dg-do run } */
 
 union V { char a; signed char b; unsigned char c; };
 struct T { char a; int b; union V c; };

gcc/testsuite/c-c++-common/torture/builtin-clear-padding-4.c

@@ -1,6 +1,6 @@
-/* { dg-require-effective-target alloca } */
-
 /* PR libstdc++/88101 */
+/* { dg-do run } */
+/* { dg-require-effective-target alloca } */
 
 struct S { char a; short b; char c; };
 

gcc/testsuite/c-c++-common/torture/builtin-clear-padding-5.c

@@ -1,4 +1,5 @@
 /* PR libstdc++/88101 */
+/* { dg-do run } */
 
 struct S { char a; short b; char c; } s1[24], s2[24];
 struct T { char a; long long b; char c; struct S d[3]; long long e; char f; } t1, t2;

gcc/testsuite/c-c++-common/torture/builtin-clear-padding-6.c

@@ -0,0 +1,28 @@
+/* PR middle-end/115527 */
+/* { dg-do run } */
+
+struct T { struct S { double a; signed char b; long c; } d[3]; int e; } t1, t2;
+
+__attribute__((noipa)) void
+foo (struct T *t)
+{
+  for (int i = 0; i < 3; ++i)
+    {
+      t->d[i].a = 1. + 3 * i;
+      t->d[i].b = 2 + 3 * i;
+      t->d[i].c = 3 + 3 * i;
+    }
+  t->e = 10;
+}
+
+int
+main ()
+{
+  __builtin_memset (&t2, -1, sizeof (t2));
+  foo (&t1);
+  foo (&t2);
+  __builtin_clear_padding (&t2);
+  if (__builtin_memcmp (&t1, &t2, sizeof (t1)))
+    __builtin_abort ();
+  return 0;
+}