[gcc r14-4722] cse: Workaround GCC < 5 bug in cse_insn [PR111852]

[gcc r14-4722] cse: Workaround GCC < 5 bug in cse_insn [PR111852]

[Jakub Jelinek]

https://gcc.gnu.org/g:bc4bd69faf986326f6b0fd0400cdd6871577afd1

commit r14-4722-gbc4bd69faf986326f6b0fd0400cdd6871577afd1
Author: Jakub Jelinek <jakub@redhat.com>
Date:   Wed Oct 18 17:01:26 2023 +0200

    cse: Workaround GCC < 5 bug in cse_insn [PR111852]
    
    Before the r5-3834 commit for PR63362, GCC 4.8-4.9 refuses to compile
    cse.cc which contains a variable with rtx_def type, because rtx_def
    contains a union with poly_uint16 element.  poly_int template has
    defaulted default constructor and a variadic template constructor which
    could have empty parameter pack. GCC < 5 treated it as non-trivially
    constructible class and deleted rtunion and rtx_def default constructors.
    
    For the cse_insn purposes, all we need is a variable with size and alignment
    of rtx_def, not necessarily rtx_def itself, which we then memset to 0 and
    fill in like rtx is normally allocated from heap, so this patch for
    GCC_VERSION < 5000 uses an unsigned char array of the right size/alignment.
    
    2023-10-18  Jakub Jelinek  <jakub@redhat.com>
    
            PR bootstrap/111852
            * cse.cc (cse_insn): Add workaround for GCC 4.8-4.9, instead of
            using rtx_def type for memory_extend_buf, use unsigned char
            arrayy with size of rtx_def and its alignment.

Diff:
---
 gcc/cse.cc | 7 +++++++
 1 file changed, 7 insertions(+)

diff --git a/gcc/cse.cc b/gcc/cse.cc
index c46870059e6c..f9603fdfd437 100644
--- a/gcc/cse.cc
+++ b/gcc/cse.cc
@@ -4951,8 +4951,15 @@ cse_insn (rtx_insn *insn)
 	  && is_a <scalar_int_mode> (mode, &int_mode)
 	  && (extend_op = load_extend_op (int_mode)) != UNKNOWN)
 	{
+#if GCC_VERSION >= 5000
 	  struct rtx_def memory_extend_buf;
 	  rtx memory_extend_rtx = &memory_extend_buf;
+#else
+	  /* Workaround GCC < 5 bug, fixed in r5-3834 as part of PR63362
+	     fix.  */
+	  alignas (rtx_def) unsigned char memory_extended_buf[sizeof (rtx_def)];
+	  rtx memory_extend_rtx = (rtx) &memory_extended_buf[0];
+#endif
 
 	  /* Set what we are trying to extend and the operation it might
 	     have been extended with.  */