C backend: fix array out of bounds warning when compiled with -O2 or higher optimizations

- see github # 217 - warning: array subscript -1 is outside array bounds
- caused by fbc's optimization of non-zero lower bound arrays
- fbc calculates a kind of virtual pointer of where the array(0,..) element would be and then computes element addresses from that
- this optimization in fbc saves some computations when accessing array elements but the translation to C is technically undefined behaviour
- fixed (for now) by casting the address to the equivalent of intptr_t in C.  (FB_DATATYPE_INTEGER).

Author: jayrm

changelog.txt

@@ -133,6 +133,7 @@ Version 1.08.0
 - fix __FB_EVAL__() incorrectly reading past the end of the expression, and report errors in expressions
 - C backend: switch to .text section after writing the exports to the C file in the explicit asm block.  gcc can move sections around with optimizations and there is a change between 7.x and 8.x that causes issue with where the directive section is located
 - sf.net #917: optimize 'm += s' string concatenations to fix the long compile times in the gcc backend (which makes heavy use of string building).
+- github #217: C backend, fix gcc array out of bounds warning when compiled with -O2 or higher optimizations and accessing non-zero lower bound fixed length string arrays
 
 
 Version 1.07.0

src/compiler/ir-hlc.bas

@@ -2678,8 +2678,23 @@ private function exprNewVREG _
 			l = exprNewUOP( AST_OP_ADDROF, l )
 		end if
 		if( have_offset ) then
-			'' Cast to ubyte ptr to work around C's pointer arithmetic
-			l = exprNewCAST( typeAddrOf( FB_DATATYPE_UBYTE ), NULL, l )
+			if( is_c_array ) then
+				'' Cast to intptr_t to work around gcc out side of array bounds
+				'' warnings if we are casting from FBSTRING array to pointer
+				'' fbc uses a kind of virtual pointer for the an array's (0,..)
+				'' index; technically this is undefinded behaviour in C and is
+				'' impossible to cast away even when using pointer only casts
+				'' in the same expression.  Some gcc optimizations cause a 
+				'' a warning when setting a pointer for the array's virtual
+				'' index location.  To fix this for compliant C code, would
+				'' need to rewrite the array descriptor to contain only the
+				'' offset value from actual memory pointer and compute the
+				'' array access fully on each array element access.   
+				l = exprNewCAST( FB_DATATYPE_INTEGER, NULL, l )
+			else
+				'' Cast to ubyte ptr to work around C's pointer arithmetic
+				l = exprNewCAST( typeAddrOf( FB_DATATYPE_UBYTE ), NULL, l )
+			end if
 			if( vreg->vidx <> NULL ) then
 				l = exprNewBOP( AST_OP_ADD, l, exprNewVREG( vreg->vidx ) )
 			end if