Optimization for MDEV-10411 Providing compatibility for basic PL/SQL constructs

When processing an SP body:

CREATE PROCEDURE p1 (parameters)
AS [ declarations ]
BEGIN statements
[ EXCEPTION exceptions ]
END;

the parser generates two "jump" instructions:
- from the end of "declarations" to the beginning of EXCEPTION
- from the end of EXCEPTION to "statements"

These jumps are useless if EXCEPTION does not exist.
This patch makes sure that these two "jump" instructions are
generated only if EXCEPTION really exists.

mysql-test/suite/compat/oracle/r/sp-code.result

@@ -21,7 +21,7 @@ END;
 /
 SHOW PROCEDURE CODE p1;
 Pos	Instruction
-0	jump 3
+0	jump 2
 CALL p1;
 DROP PROCEDURE p1;
 # No HANDLER declarations, no code, some exceptions
@@ -133,10 +133,9 @@ SHOW PROCEDURE CODE p1;
 Pos	Instruction
 0	hpush_jump 3 1 EXIT
 1	set v@0 123
-2	hreturn 0 5
+2	hreturn 0 4
 3	set v@0 223
-4	jump 5
-5	hpop 1
+4	hpop 1
 set @v= 10;
 CALL p1(@v);
 SELECT @v;

sql/sp_head.cc

@@ -2582,6 +2582,34 @@ bool sp_head::add_instr_jump_forward_with_backpatch(THD *thd,
 }
 
 
+/*
+  Replace an instruction at position to "no operation".
+
+  @param thd - use mem_root of this THD for "new".
+  @param ip  - position of the operation
+  @returns   - true on error, false on success
+
+  When we need to remove an instruction that during compilation
+  appeared to be useless (typically as useless jump), we replace
+  it to a jump to exactly the next instruction.
+  Such jumps are later removed during sp_head::optimize().
+
+  QQ: Perhaps we need a dedicated sp_instr_nop for this purpose.
+*/
+bool sp_head::replace_instr_to_nop(THD *thd, uint ip)
+{
+  sp_instr *instr= get_instr(ip);
+  sp_instr_jump *nop= new (thd->mem_root) sp_instr_jump(instr->m_ip,
+                                                        instr->m_ctx,
+                                                        instr->m_ip + 1);
+  if (!nop)
+    return true;
+  delete instr;
+  set_dynamic(&m_instr, (uchar *) &nop, ip);
+  return false;
+}
+
+
 /**
   Do some minimal optimization of the code:
     -# Mark used instructions

sql/sp_head.h

@@ -367,6 +367,8 @@ class sp_head :private Query_arena,
     return i;
   }
 
+  bool replace_instr_to_nop(THD *thd, uint ip);
+
   /*
     Resets lex in 'thd' and keeps a copy of the old one.
 

sql/sql_lex.cc

@@ -5428,8 +5428,20 @@ LEX::sp_block_with_exceptions_finalize_executable_section(THD *thd,
 
 bool
 LEX::sp_block_with_exceptions_finalize_exceptions(THD *thd,
-                                                  uint executable_section_ip)
+                                                  uint executable_section_ip,
+                                                  uint exception_count)
 {
+  if (!exception_count)
+  {
+    /*
+      The jump from the end of DECLARE section to
+      the beginning of the EXCEPTION section that we added in
+      sp_block_with_exceptions_finalize_declarations() is useless
+      if there were no exceptions.
+      Replace it to "no operation".
+    */
+    return sphead->replace_instr_to_nop(thd, executable_section_ip - 1);
+  }
   /*
     Generate a jump from the end of the EXCEPTION code
     to the executable section.

sql/sql_lex.h

@@ -3149,7 +3149,8 @@ struct LEX: public Query_tables_list
   bool sp_block_with_exceptions_finalize_executable_section(THD *thd,
                                                   uint executable_section_ip);
   bool sp_block_with_exceptions_finalize_exceptions(THD *thd,
-                                                  uint executable_section_ip);
+                                                  uint executable_section_ip,
+                                                  uint exception_count);
   // Check if "KEY IF NOT EXISTS name" used outside of ALTER context
   bool check_add_key(DDL_options_st ddl)
   {

sql/sql_yacc_ora.yy

@@ -3489,7 +3489,7 @@ sp_block_statements_and_exceptions:
           }
           opt_exception_clause
           {
-            if (Lex->sp_block_with_exceptions_finalize_exceptions(thd, $1))
+            if (Lex->sp_block_with_exceptions_finalize_exceptions(thd, $1, $4))
               MYSQL_YYABORT;
             $$= $4;
           }