[libm] Avoid left shift of signed integer entities

Follow-up commit d180086e6e by fixing the left shift of signed integer
entities through the use of a helper function. Specific per file changes
are:

* lib/msun/src/e_fmodf.c:
* lib/msun/src/s_remquof.c:
  . Eliminate now unused variable 'i'.
  . Sort declaration statement.
  . Use subnormal_ilogbf() to avoid left shift of signed integer.

* lib/msun/src/math_private.h b/lib/msun/src/math_private.h:
  . Implement subnormal_ilogbf() to extract an exponent of a subnormal
    float.  This avoids left shifts of signed integers.
  . Update nearby comment.

* lib/msun/src/s_ilogbf.c
  . Fix declaration of the function statement in accordance with style(9).
  . Use subnormal_ilogbf() to avoid left shift of signed integer.

PR:		288850
MFC after:	1 week

lib/msun/src/e_fmodf.c

@@ -27,7 +27,7 @@ static const float one = 1.0, Zero[] = {0.0, -0.0,};
 float
 fmodf(float x, float y)
 {
-	int32_t n,hx,hy,hz,ix,iy,sx,i;
+	int32_t hx, hy, hz, ix, iy, n, sx;
 
 	GET_FLOAT_WORD(hx,x);
 	GET_FLOAT_WORD(hy,y);
@@ -44,14 +44,16 @@ fmodf(float x, float y)
 	    return Zero[(u_int32_t)sx>>31];	/* |x|=|y| return x*0*/
 
     /* determine ix = ilogb(x) */
-	if(hx<0x00800000) {	/* subnormal x */
-	    for (ix = -126,i=(hx<<8); i>0; i<<=1) ix -=1;
-	} else ix = (hx>>23)-127;
+	if(hx<0x00800000)
+	    ix = subnormal_ilogbf(hx);
+	else
+	    ix = (hx>>23)-127;
 
     /* determine iy = ilogb(y) */
-	if(hy<0x00800000) {	/* subnormal y */
-	    for (iy = -126,i=(hy<<8); i>=0; i<<=1) iy -=1;
-	} else iy = (hy>>23)-127;
+	if(hy<0x00800000)
+	    iy = subnormal_ilogbf(hy);
+	else
+	    iy = (hy>>23)-127;
 
     /* set up {hx,lx}, {hy,ly} and align y to x */
 	if(ix >= -126)

lib/msun/src/math_private.h

@@ -740,7 +740,7 @@ irintl(long double x)
 } while (0)
 
 /*
- * For a double entity split into high and low parts, compute ilogb.
+ * For a subnormal double entity split into high and low parts, compute ilogb.
  */
 static inline int32_t
 subnormal_ilogb(int32_t hi, int32_t lo)
@@ -760,6 +760,20 @@ subnormal_ilogb(int32_t hi, int32_t lo)
 	return (j);
 }
 
+/*
+ * For a subnormal float entity represented as an int32_t, compute ilogb.
+ */
+static inline int32_t
+subnormal_ilogbf(int32_t hx)
+{
+	int32_t j;
+	uint32_t i;
+	i = (uint32_t) hx << 8;
+	for (j = -126; i < 0x7fffffff; i <<= 1) j -=1;
+
+	return (j);
+}
+
 #ifdef DEBUG
 #if defined(__amd64__) || defined(__i386__)
 #define	breakpoint()	asm("int $3")

lib/msun/src/s_ilogbf.c

@@ -18,7 +18,8 @@
 #include "math.h"
 #include "math_private.h"
 
-	int ilogbf(float x)
+int
+ilogbf(float x)
 {
 	int32_t hx,ix;
 
@@ -28,7 +29,7 @@
 	    if(hx==0)
 		return FP_ILOGB0;
 	    else			/* subnormal x */
-	        for (ix = -126,hx<<=8; hx>0; hx<<=1) ix -=1;
+		ix = subnormal_ilogbf(hx);
 	    return ix;
 	}
 	else if (hx<0x7f800000) return (hx>>23)-127;

lib/msun/src/s_remquof.c

@@ -25,7 +25,7 @@ static const float Zero[] = {0.0, -0.0,};
 float
 remquof(float x, float y, int *quo)
 {
-	int32_t n,hx,hy,hz,ix,iy,sx,i;
+	int32_t hx, hy, hz, ix, iy, n, sx;
 	u_int32_t q,sxy;
 
 	GET_FLOAT_WORD(hx,x);
@@ -47,14 +47,16 @@ remquof(float x, float y, int *quo)
 	}
 
     /* determine ix = ilogb(x) */
-	if(hx<0x00800000) {	/* subnormal x */
-	    for (ix = -126,i=(hx<<8); i>0; i<<=1) ix -=1;
-	} else ix = (hx>>23)-127;
+	if(hx<0x00800000)
+	    ix = subnormal_ilogbf(hx);
+	else
+	    ix = (hx>>23)-127;
 
     /* determine iy = ilogb(y) */
-	if(hy<0x00800000) {	/* subnormal y */
-	    for (iy = -126,i=(hy<<8); i>0; i<<=1) iy -=1;
-	} else iy = (hy>>23)-127;
+	if(hy<0x00800000)
+	    iy = subnormal_ilogbf(hy);
+	else
+	    iy = (hy>>23)-127;
 
     /* set up {hx,lx}, {hy,ly} and align y to x */
 	if(ix >= -126)