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Fix PPC64 JIT conditional branch offset overflow for large QVM files #387

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Fix PPC64 JIT conditional branch offset overflow for large QVM files #387

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36 changes: 8 additions & 28 deletions code/qcommon/vm_powerpc.c
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Original file line number Diff line number Diff line change
Expand Up @@ -1292,41 +1292,21 @@ static void get_branch_cond( int op, int *bo, int *bi )
}


#if 0
static void emit_branchConditional( vm_t *vm, instruction_t *ci, int op )
{
int32_t target = ci->value; // target instruction index
int32_t targetOfs = vm->instructionPointers[ target ] - compiledOfs;
int bo, bi;

get_branch_cond( op, &bo, &bi );

// Long form: inverted bc skips over unconditional b
// Invert: BO_TRUE(12) <-> BO_FALSE(4)
{
int inv_bo = ( bo == BO_TRUE ) ? BO_FALSE : BO_TRUE;
emit( PPC_BC( inv_bo, bi, +8 ) ); // skip next instruction if NOT condition
emit( PPC_B( targetOfs - 4 ) ); // -4 because compiledOfs advanced by 4
}
}
#endif


static void emit_branchConditionalShort( vm_t* vm, instruction_t* ci )
{
int32_t targetOfs = vm->instructionPointers[ ci->value ] - compiledOfs;
int bo, bi;
int inv_bo;

get_branch_cond( ci->op, &bo, &bi );

if ( pass != PASS_INIT ) {
if ( (int16_t)targetOfs != targetOfs ) {
// TODO: add/switch to expansion pass?
DROP( "offset is too large" );
}
}

emit( PPC_BC( bo, bi, targetOfs ) );
// PPC64 bc instruction only supports 16-bit signed offsets (±32KB).
// For large QVM files (e.g. cgame.qvm) branch targets can exceed this.
// Always use long form: inverted bc to skip over unconditional b (26-bit offset).
// This ensures consistent code size across compilation passes.
inv_bo = ( bo == BO_TRUE ) ? BO_FALSE : BO_TRUE;
emit( PPC_BC( inv_bo, bi, +8 ) ); // skip next instruction if NOT condition
emit( PPC_B( targetOfs - 4 ) ); // -4 because compiledOfs advanced by 4
}


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