#3999 Out-of-range branch generated by SDCC

[Michel Iwaniec]

When compiling with the latest nightly build for mos6502, the compiler sometimes generates an .asm file which has an out-of-range branch address.
In these cases it would be expected to add an unconditional jmp instruction, and a branch with the opposite condition.

Tested with 4.6.0 #16555 (MINGW64)
(sdcc-snapshot-x86_64-w64-mingw32-20260602-16555)

Building with:

sdcc -mmos6502 -c out_of_range_branch_codegen.c

Error

out_of_range_branch_codegen.asm:147: Error: <a> machine specific addressing or addressing mode error

Minimal example to reproduce:

//
// Generates a codegen error for 6502: A branch instruction trying to jump beyond a signed 8-bit relative PC value
//
#include <stdint.h>

uint16_t X, Y;
int16_t Z;

uint8_t f(uint16_t x)
{
    return x + 42;
}

void main(void)
{
    uint16_t x = X >> 4;
    if(Z > 0)
    {
        if(f(x) || f(x+1) || f(x+2) || f(x+3) || f(x+4) || f(x+5) || f(x+6))
        {
            return;
        }
    }
}

[Philipp Klaus Krause]

I can reproduce the issue on my Debian GNU/Linux testing system using sdcc built from current trunk.

[bbbbbr]

Just subscribing to this ticket

[Gabriele Gorla]

bug is confirmed. I have a fix. I'll send a mail to the devel list to confirm it's ok to check-in before 4.6.0 release.

[Gabriele Gorla]

Index: src/mos6502/gen.c
===================================================================
--- src/mos6502/gen.c   (revision 16596)
+++ src/mos6502/gen.c   (working copy)
@@ -5361,6 +5361,7 @@
   symbol *true_lbl = NULL;
   symbol *false_lbl = NULL;
   bool needloada = false;

+  symbol *skiplbl = NULL;

   opcode = ic->op;

@@ -5415,6 +5416,8 @@

   if (ifx)
     {

+      skiplbl = m6502_safeNewiTempLabel (NULL);
+
       if (IC_TRUE (ifx))
         {
           ifx_jmp_lbl = IC_TRUE (ifx);
@@ -5452,11 +5455,11 @@

       if(ifx)
         {

-          symbol *skiplbl = m6502_safeNewiTempLabel (NULL);
+          symbol *skiplbl1 = m6502_safeNewiTempLabel (NULL);


-          m6502_emitBranch ("bpl", skiplbl);
+          m6502_emitBranch ("bpl", skiplbl1);
                                  m6502_emitBranch ("jmp", ifx_jmp_lbl);
-          m6502_safeEmitLabel (skiplbl);
+          m6502_safeEmitLabel (skiplbl1);
         }
       else
         {
@@ -5467,7 +5470,7 @@
       if(size==2)
         {
           if(ifx)
-            m6502_emitBranch ("bne", ifx_jmp_lbl);
+            m6502_emitBranch ("bne", skiplbl);
           else
             m6502_emitBranch ("bne", true_lbl);

@@ -5607,8 +5610,6 @@

   if (ifx)
     {

-      symbol *skiplbl = m6502_safeNewiTempLabel (NULL);
-
       if(needloada)
         {
      if (!strcmp(br_inst, "bcc") || !strcmp(br_inst, "bcs"))

quick description:
There was a logic bug in the original code. ifx_jmp_lbl was used instead of skiplbl
move skiplbl definition at the beginning of the function.
rename skiplbl to skiplbl1 for the first byte.
use skiplbl instead of ifx_jmp_lbl

[Gabriele Gorla]

fixed in [r16601]