fixed ldexpf underflow behaviour for exponent of 0

Author: ZERICO2005

src/libc/ldexpf.src

@@ -47,9 +47,11 @@ _scalbn:
 	bit	7, (iy + 11)	; scale sign
 	jr	z, .scale_up
 .scale_down:
-	; test signbit
+	; HL is not INT_MIN here
+	dec	hl
 	add	hl, hl
-	jr	nc, .finish
+	jr	nc, .finish	; expon > 0
+	; expon <= 0 or subnormal
 .underflow_to_zero:
 	ld	hl, ___fe_cur_env
 	set	5, (hl)	; FE_INEXACT
@@ -99,7 +101,6 @@ end if
 	ld	e, (iy + 6)
 	ret
 
-
 else
 
 ; normal inputs are handled correctly, unless the output is subnormal
@@ -132,9 +133,12 @@ _scalbn:
 .scale_down:
 	; test signbit
 	push	hl
+	; HL is not INT_MIN here
+	dec	hl
 	add	hl, hl
 	pop	hl
-	jr	nc, .finish
+	jr	nc, .finish	; expon > 0
+	; expon <= 0 or subnormal
 ;	jr	.underflow_to_zero
 .underflow_to_zero:
 	ld	hl, ___fe_cur_env

test/floating_point/float32_ldexp/src/main.c

@@ -29,20 +29,16 @@ size_t run_test(void) {
     for (size_t i = 0; i < length; i++) {
         F32_pun result;
         result.flt = ldexpf(input[i].value, input[i].expon);
-        // ignoring subnormal inputs for now
-        if (issubnormal(input[i].value) || issubnormal(output[i].flt)) {
-            continue;
-        }
         if (result.bin != output[i].bin) {
             // ignore NaN's with differing payloads
             // treat signed zeros as equal for now
             if (
                 (!(isnan(result.flt) && isnan(output[i].flt))) &&
-                (!(iszero(result.flt) && iszero(output[i].flt)))
+                (!(result.bin == 0 && iszero(output[i].flt)))
             ) {
                 /* Float multiplication does not handle subnormals yet */
-                if (!(iszero(result.flt) && issubnormal(output[i].flt))) {
-                    #if 0
+                if (!(iszero(result.flt) && (issubnormal(output[i].flt) || issubnormal(input[i].value)))) {
+                    #if 1
                         printf(
                             "%zu:\nI: %08lX %+d\nG: %08lX\nT: %08lX\n",
                             i, *(uint32_t*)(void*)&(input[i].value), input[i].expon,