Add flag to rgbasm to disable LD->LDH optimization

rgbasm tries to optimize any loads from/to $FF00-$FFFF and generate
LDH 2-byte opcodes instead of regular LD 3-byte opcodes. This is a bit
inconsistent as it only works for constant values. If a load is trying
to access a label in a HRAM floating section, or a section found in a
different object file, this optimization doesn't work.

This means that a simple refactor or code could allow rgbasm to perform
the optimzation or prevent it from doing so. For certain projects, like
disassemblies, this is a problem.

This patch adds flag -L to rgbasm to disable the optimization, and
doesn't change the behaviour of any other existing code.

Signed-off-by: Antonio Niño Díaz <antonio_nd@outlook.com>

src/asm/asmy.y

@@ -1728,7 +1728,8 @@ z80_ld_mem	: T_Z80_LD op_mem_ind comma T_MODE_SP
 		}
 		| T_Z80_LD op_mem_ind comma T_MODE_A
 		{
-			if ((!rpn_isReloc(&$2)) && ($2.nVal >= 0xFF00)) {
+			if (CurrentOptions.optimizeloads &&
+			    (!rpn_isReloc(&$2)) && ($2.nVal >= 0xFF00)) {
 				out_AbsByte(0xE0);
 				out_AbsByte($2.nVal & 0xFF);
 			} else {
@@ -1781,7 +1782,8 @@ z80_ld_a	: T_Z80_LD reg_r comma T_MODE_C_IND
 		| T_Z80_LD reg_r comma op_mem_ind
 		{
 			if ($2 == REG_A) {
-				if ((!rpn_isReloc(&$4)) && ($4.nVal >= 0xFF00)) {
+				if (CurrentOptions.optimizeloads &&
+				    (!rpn_isReloc(&$4)) && ($4.nVal >= 0xFF00)) {
 					out_AbsByte(0xF0);
 					out_AbsByte($4.nVal & 0xFF);
 				} else {

src/asm/main.c

@@ -282,7 +282,7 @@ void warning(const char *fmt, ...)
 static void print_usage(void)
 {
 	printf(
-"usage: rgbasm [-EhVvw] [-b chars] [-Dname[=value]] [-g chars] [-i path]\n"
+"usage: rgbasm [-EhLVvw] [-b chars] [-Dname[=value]] [-g chars] [-i path]\n"
 "              [-M dependfile] [-o outfile] [-p pad_value] file.asm\n");
 	exit(1);
 }
@@ -316,17 +316,18 @@ int main(int argc, char *argv[])
 	DefaultOptions.gbgfx[3] = '3';
 	DefaultOptions.binary[0] = '0';
 	DefaultOptions.binary[1] = '1';
-	DefaultOptions.fillchar = 0;
-	DefaultOptions.verbose = false;
-	DefaultOptions.haltnop = true;
 	DefaultOptions.exportall = false;
+	DefaultOptions.fillchar = 0;
+	DefaultOptions.optimizeloads = true;
+	DefaultOptions.haltnop = true;
+	DefaultOptions.verbose = false;
 	DefaultOptions.warnings = true;
 
 	opt_SetCurrentOptions(&DefaultOptions);
 
 	newopt = CurrentOptions;
 
-	while ((ch = getopt(argc, argv, "b:D:g:hi:M:o:p:EVvw")) != -1) {
+	while ((ch = getopt(argc, argv, "b:D:Eg:hi:LM:o:p:Vvw")) != -1) {
 		switch (ch) {
 		case 'b':
 			if (strlen(optarg) == 2) {
@@ -358,6 +359,9 @@ int main(int argc, char *argv[])
 		case 'i':
 			fstk_AddIncludePath(optarg);
 			break;
+		case 'L':
+			newopt.optimizeloads = false;
+			break;
 		case 'M':
 			dependfile = fopen(optarg, "w");
 			if (dependfile == NULL)

src/asm/rgbasm.1

@@ -5,7 +5,7 @@
 .\"
 .\" SPDX-License-Identifier: MIT
 .\"
-.Dd January 26, 2018
+.Dd February 24, 2018
 .Dt RGBASM 1
 .Os RGBDS Manual
 .Sh NAME
@@ -13,7 +13,7 @@
 .Nd Game Boy assembler
 .Sh SYNOPSIS
 .Nm rgbasm
-.Op Fl EhVvw
+.Op Fl EhLVvw
 .Op Fl b Ar chars
 .Op Fl D Ar name Ns Op = Ns Ar value
 .Op Fl g Ar chars
@@ -55,6 +55,12 @@ The
 option disables this behavior.
 .It Fl i Ar path
 Add an include path.
+.It Fl L
+Disable the optimization that turns loads of the form
+.Sy LD [$FF00+n8],A
+into the opcode
+.Sy LDH [$FF00+n8],A
+in order to have full control of the result in the final ROM.
 .It Fl M Ar dependfile
 Print
 .Xr make 1
@@ -75,7 +81,9 @@ Disable warning output.
 .Sh EXAMPLES
 Assembling a basic source file is simple:
 .Pp
-.D1 $ rgbasm -o bar.o foo.asm
+.Bd -literal -offset indent
+$ rgbasm -o bar.o foo.asm
+.Ed
 .Pp
 The resulting object file is not yet a usable ROM image \(em it must first be
 run through

src/asm/rgbasm.5

@@ -5,7 +5,7 @@
 .\"
 .\" SPDX-License-Identifier: MIT
 .\"
-.Dd January 27, 2018
+.Dd February 24, 2018
 .Dt RGBASM 5
 .Os RGBDS Manual
 .Sh NAME
@@ -110,7 +110,13 @@ and
 .Sy LDH A,[$FF00+n8]
 syntax instead.
 This forces the assembler to emit the correct instruction and the linker to
-check if the value is in the correct range.
+check if the value is in the correct range. This optimization can be disabled
+by passing the
+.Fl L
+flag to
+.Sy rgbasm
+as explained in
+.Xr rgbasm 1 .
 .El
 .Pp
 A section is usually defined as a floating one, but the code can restrict where