Fix internal error on non-byte-aligned reference in GIMPLE DSE

This is a regression present on the mainline, 13 and 12 branches.  For the
attached Ada case, it's a tree checking failure on the mainline at -O:

+===========================GNAT BUG DETECTED==============================+
| 14.0.1 20240226 (experimental) [master r14-9171-g4972f97a265]  GCC error:|
| tree check: expected tree that contains 'decl common' structure,         |
| have 'component_ref' in tree_could_trap_p, at tree-eh.cc:2733        |
| Error detected around /home/eric/cvs/gcc/gcc/testsuite/gnat.dg/opt104.adb:

Time is a 10-byte record and Packed_Rec.T is placed at bit-offset 65 because
of the packing. so tree-ssa-dse.cc:setup_live_bytes_from_ref has computed a
const_size of 88 from ref->offset of 65 and ref->max_size of 80.

Then in tree-ssa-dse.cc:compute_trims:

411       int last_live = bitmap_last_set_bit (live);
(gdb) next
412       if (ref->size.is_constant (&const_size))
(gdb)
414           int last_orig = (const_size / BITS_PER_UNIT) - 1;
(gdb)
418           *trim_tail = last_orig - last_live;

(gdb) call debug_bitmap (live)
n_bits = 256, set = {0 1 2 3 4 5 6 7 8 9 10 }
(gdb) p last_live
$33 = 10
(gdb) p const_size
$34 = 80
(gdb) p last_orig
$35 = 9
(gdb) p *trim_tail
$36 = -1

In other words, compute_trims is overlooking the alignment adjustments that
setup_live_bytes_from_ref applied earlier.  Moveover it reads:

  /* We use sbitmaps biased such that ref->offset is bit zero and the bitmap
     extends through ref->size.  So we know that in the original bitmap
     bits 0..ref->size were true.  We don't actually need the bitmap, just
     the REF to compute the trims.  */

but setup_live_bytes_from_ref used ref->max_size instead of ref->size.

It appears that all the callers of compute_trims assume that ref->offset is
byte aligned and that the trimmed bytes are relative to ref->size, so the
patch simply adds an early return if either condition is not fulfilled.

gcc/
	* tree-ssa-dse.cc (compute_trims): Fix description.  Return early
	if either ref->offset is not byte aligned or ref->size is not known
	to be equal to ref->max_size.
	(maybe_trim_complex_store): Fix description.
	(maybe_trim_constructor_store): Likewise.
	(maybe_trim_partially_dead_store): Likewise.

gcc/testsuite/
	* gnat.dg/opt104.ads, gnat.dg/opt104.adb: New test.

2 files changed, 62 insertions, 0 deletions

diff --git a/gcc/testsuite/gnat.dg/opt104.adb b/gcc/testsuite/gnat.dg/opt104.adb
new file mode 100644
index 0000000..1548c3e
--- /dev/null
+++ b/gcc/testsuite/gnat.dg/opt104.adb
@@ -0,0 +1,22 @@
+-- { dg-do compile }
+-- { dg-options "-O -gnatws" }
+
+package body Opt104 is
+
+   procedure Proc (R : Rec) is
+      Data : Packed_Rec;
+
+   begin
+      case R.D is
+         when True =>
+            for I in 1 .. R.Len loop
+               exit;
+            end loop;
+
+         when False =>
+            null;
+      end case;
+   end;
+
+end Opt104;
+
diff --git a/gcc/testsuite/gnat.dg/opt104.ads b/gcc/testsuite/gnat.dg/opt104.ads
new file mode 100644
index 0000000..a177ed0
--- /dev/null
+++ b/gcc/testsuite/gnat.dg/opt104.ads
@@ -0,0 +1,40 @@
+package Opt104 is
+
+   type Time is record
+      S  : Integer;
+      B1 : Boolean;
+      B2 : Boolean;
+      B3 : Boolean;
+      B4 : Boolean;
+      B5 : Boolean;
+      B6 : Boolean;
+   end record;
+
+   Zero_Time : constant Time :=
+     (S  => 0,
+      B1 => False,
+      B2 => False,
+      B3 => False,
+      B4 => False,
+      B5 => False,
+      B6 => False);
+
+   type Root is tagged null record;
+
+   type Packed_Rec is record
+      R : Root;
+      B : Boolean;
+      T : Time := Zero_Time;
+   end record;
+   pragma Pack (Packed_Rec);
+
+   type Rec (D : Boolean) is record
+      case D is
+         when True  => Len : Integer;
+         when False => null;
+      end case;
+   end record;
+
+   procedure Proc (R : Rec);
+
+end Opt104;