Re: [RFC PATCH 0/3] fix *pbl format support

[Rasmus Villemoes <linux@rasmusvillemoes.dk>]

On Wed, Sep 16 2015, Tejun Heo <tj@kernel.org> wrote:

>
> I suppose (2) could work too but we really should strive to provide
> something convenient to print[fk] users.  The balance here is pretty
> one-sided.
>

I just remembered: I noticed a while ago that the qualifier member is
only used inside format_decode (in the end, the information is folded
into the type member), so one might as well use a local variable for
that. This gives option (3): Make field_width a 24 bit bitfield. I think
that should be sufficient for any realistic bitmap that one would
print. The patch is also rather small. Surprisingly, bloat-o-meter even
says that it's also a net win in code size:

add/remove: 18/16 grow/shrink: 3/2 up/down: 5551/-5775 (-224)

(the huge absolute numbers are due to stuff like 

pointer                                        -    1148   +1148
pointer.isra                                1116       -   -1116

so the functions haven't actually changed that much). I'll have to check
how this can be (smaller might still be worse), but at least it doesn't
seem to be a catastrophe in terms of .text bloat.

[Grr, why don't we have a way to do a compile-time assert outside
function context?]

Only compile-tested.

From: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Date: Wed, 16 Sep 2015 22:06:14 +0200
Subject: [RFC] lib/vsprintf.c: expand field_width to 24 bits

Maurizio Lombardi reported a problem with the %pb extension: It
doesn't work for sufficiently large bitmaps, since the size is stashed
in the field_width field of the struct printf_spec, which is currently
an s16. Concretely, this manifested itself in
/sys/bus/pseudo/drivers/scsi_debug/map being empty, since the bitmap
printer got a size of 0, which is the 16 bit truncation of the actual
bitmap size.

We do want to keep struct printf_spec at 8 bytes so that it can
cheaply be passed by value. The qualifier field is only used for
internal bookkeeping in format_decode, so we might as well use a local
variable for that. This gives us an additional 8 bits, which we can
then use for the field width. To stay in 8 bytes, we need to do a
little rearranging and make the type member a bitfield as well.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
---
 lib/vsprintf.c | 33 +++++++++++++++++----------------
 1 file changed, 17 insertions(+), 16 deletions(-)

diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 95cd63b43b99..cce2a780a82e 100644
--- a/lib/vsprintf.c
+++ b/lib/vsprintf.c
@@ -380,13 +380,13 @@ enum format_type {
 };
 
 struct printf_spec {
-	u8	type;		/* format_type enum */
+	u8	type:8;		/* format_type enum */
+	s32	field_width:24;	/* width of output field */
 	u8	flags;		/* flags to number() */
 	u8	base;		/* number base, 8, 10 or 16 only */
-	u8	qualifier;	/* number qualifier, one of 'hHlLtzZ' */
-	s16	field_width;	/* width of output field */
 	s16	precision;	/* # of digits/chars */
 };
+extern char __check_printf_spec[1-2*(sizeof(struct printf_spec) != 8)];
 
 static noinline_for_stack
 char *number(char *buf, char *end, unsigned long long num,
@@ -1633,6 +1633,7 @@ static noinline_for_stack
 int format_decode(const char *fmt, struct printf_spec *spec)
 {
 	const char *start = fmt;
+	char qualifier;
 
 	/* we finished early by reading the field width */
 	if (spec->type == FORMAT_TYPE_WIDTH) {
@@ -1715,16 +1716,16 @@ precision:
 
 qualifier:
 	/* get the conversion qualifier */
-	spec->qualifier = -1;
+	qualifier = 0;
 	if (*fmt == 'h' || _tolower(*fmt) == 'l' ||
 	    _tolower(*fmt) == 'z' || *fmt == 't') {
-		spec->qualifier = *fmt++;
-		if (unlikely(spec->qualifier == *fmt)) {
-			if (spec->qualifier == 'l') {
-				spec->qualifier = 'L';
+		qualifier = *fmt++;
+		if (unlikely(qualifier == *fmt)) {
+			if (qualifier == 'l') {
+				qualifier = 'L';
 				++fmt;
-			} else if (spec->qualifier == 'h') {
-				spec->qualifier = 'H';
+			} else if (qualifier == 'h') {
+				qualifier = 'H';
 				++fmt;
 			}
 		}
@@ -1781,19 +1782,19 @@ qualifier:
 		return fmt - start;
 	}
 
-	if (spec->qualifier == 'L')
+	if (qualifier == 'L')
 		spec->type = FORMAT_TYPE_LONG_LONG;
-	else if (spec->qualifier == 'l') {
+	else if (qualifier == 'l') {
 		BUILD_BUG_ON(FORMAT_TYPE_ULONG + SIGN != FORMAT_TYPE_LONG);
 		spec->type = FORMAT_TYPE_ULONG + (spec->flags & SIGN);
-	} else if (_tolower(spec->qualifier) == 'z') {
+	} else if (_tolower(qualifier) == 'z') {
 		spec->type = FORMAT_TYPE_SIZE_T;
-	} else if (spec->qualifier == 't') {
+	} else if (qualifier == 't') {
 		spec->type = FORMAT_TYPE_PTRDIFF;
-	} else if (spec->qualifier == 'H') {
+	} else if (qualifier == 'H') {
 		BUILD_BUG_ON(FORMAT_TYPE_UBYTE + SIGN != FORMAT_TYPE_BYTE);
 		spec->type = FORMAT_TYPE_UBYTE + (spec->flags & SIGN);
-	} else if (spec->qualifier == 'h') {
+	} else if (qualifier == 'h') {
 		BUILD_BUG_ON(FORMAT_TYPE_USHORT + SIGN != FORMAT_TYPE_SHORT);
 		spec->type = FORMAT_TYPE_USHORT + (spec->flags & SIGN);
 	} else {
-- 
2.1.3