[gcc r14-6209] lower-bitint: Fix arithmetics followed by extension by many bits [PR112809]

[Jakub Jelinek <jakub@gcc.gnu.org>]

https://gcc.gnu.org/g:0ca64f846edce3c7b7f26bcc5978118e560e65b1

commit r14-6209-g0ca64f846edce3c7b7f26bcc5978118e560e65b1
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Dec 6 09:55:30 2023 +0100

    lower-bitint: Fix arithmetics followed by extension by many bits [PR112809]
    
    A zero or sign extension from result of some upwards_2limb operation
    is implemented in lower_mergeable_stmt as an extra loop which fills in
    the extra bits with 0s or 1s.
    If the delta of extended vs. unextended bit count is small, the code
    doesn't use a loop and emits up to a couple of stores to constant indexes,
    but if the delta is large, it uses
              cnt = (bo_bit != 0) + 1 + (rem != 0);
    statements.  bo_bit is non-zero for bit-field loads and is done in that
    case as straight line, the unconditional 1 in there is for a loop which
    handles most of the limbs in the delta and finally (rem != 0) is for the
    case when the extended precision is not a multiple of limb_prec and is
    again done in straight line code (after the loop).
    The testcase ICEs because the decision what idx to use was incorrect
    for kind == bitint_prec_huge (i.e. when the precision delta is very large)
    and rem == 0 (i.e. the extended precision is multiple of limb_prec).
    In that case cnt is either 1 (if bo_bit == 0) or 2, and idx should
    be either first size_int (start) and then result of create_loop (for bo_bit
    != 0) or just result of create_loop, but by mistake the last case
    was size_int (end), which means when precision is multiple of limb_prec
    storing above the precision (which ICEs; but also not emitting the loop
    which is needed).
    
    2023-12-06  Jakub Jelinek  <jakub@redhat.com>
    
            PR tree-optimization/112809
            * gimple-lower-bitint.cc (bitint_large_huge::lower_mergeable_stmt): For
            separate_ext in kind == bitint_prec_huge mode if rem == 0, create for
            i == cnt - 1 the loop rather than using size_int (end).
    
            * gcc.dg/bitint-48.c: New test.

Diff:
---
 gcc/gimple-lower-bitint.cc       |  2 +-
 gcc/testsuite/gcc.dg/bitint-48.c | 23 +++++++++++++++++++++++
 2 files changed, 24 insertions(+), 1 deletion(-)

diff --git a/gcc/gimple-lower-bitint.cc b/gcc/gimple-lower-bitint.cc
index 575883a072d..d2026f6c905 100644
--- a/gcc/gimple-lower-bitint.cc
+++ b/gcc/gimple-lower-bitint.cc
@@ -2624,7 +2624,7 @@ bitint_large_huge::lower_mergeable_stmt (gimple *stmt, tree_code &cmp_code,
 	{
 	  if (kind == bitint_prec_large || (i == 0 && bo_bit != 0))
 	    idx = size_int (start + i);
-	  else if (i == cnt - 1)
+	  else if (i == cnt - 1 && (rem != 0))
 	    idx = size_int (end);
 	  else if (i == (bo_bit != 0))
 	    idx = create_loop (size_int (start + i), &idx_next);
diff --git a/gcc/testsuite/gcc.dg/bitint-48.c b/gcc/testsuite/gcc.dg/bitint-48.c
new file mode 100644
index 00000000000..8701ebb9b6e
--- /dev/null
+++ b/gcc/testsuite/gcc.dg/bitint-48.c
@@ -0,0 +1,23 @@
+/* PR tree-optimization/112809 */
+/* { dg-do compile { target bitint } } */
+/* { dg-options "-O2" } */
+
+#if __BITINT_MAXWIDTH__ >= 512
+_BitInt (512) a;
+_BitInt (256) b;
+_BitInt (256) c;
+
+int
+foo (void)
+{
+  return a == (b | c);
+}
+
+void
+bar (void)
+{
+  a /= b - 2;
+}
+#else
+int i;
+#endif