chris@mimsy.UUCP (Chris Torek) (04/15/88)
In article <5206@ihlpg.ATT.COM> tainter@ihlpg.ATT.COM (Tainter) writes:
-Is
- "ABCD"[0]
-legal ANSI C?
Yes. It is not, however, a constant expression (see section 3.4).
-If so then
- (#x[0])
-becomes a 'charize' expression equivalent to a direct charize operator for
-all intents and purposes.
No. For instance, "C"[0] is not legal as a case label.
--
In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163)
Domain: chris@mimsy.umd.edu Path: uunet!mimsy!christainter@ihlpg.ATT.COM (Tainter) (04/18/88)
In article <11072@mimsy.UUCP>, chris@mimsy.UUCP (Chris Torek) writes: > In article <5206@ihlpg.ATT.COM> tainter@ihlpg.ATT.COM (Tainter) writes: > -Is "ABCD"[0] legal ANSI C? > Yes. It is not, however, a constant expression (see section 3.4). Well then, fix the standard! Quick before it gets cast in concrete! There is no possbile excuse for this not being a constant expression. Every component is a constant, why isn't the result a constant? > In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) > Domain: chris@mimsy.umd.edu Path: uunet!mimsy!chris
chris@trantor.umd.edu (Chris Torek) (04/18/88)
>>In article <5206@ihlpg.ATT.COM> tainter@ihlpg.ATT.COM (Tainter) asks: >>-Is "ABCD"[0] legal ANSI C? >In article <11072@mimsy.UUCP> I answered: >>Yes. It is not, however, a constant expression (see section 3.4). In article <5214@ihlpg.ATT.COM> tainter@ihlpg.ATT.COM (Tainter) replies: >Well then, fix the standard! Quick before it gets cast in concrete! >Every component is a constant, why isn't the result a constant? To a large extent, I happen to agree. Constants should (almost?) always be reduced to their simplest form at compile time. There is one argument against converting "x"[0] to 'x' at compile time; it is rather weak: Some C compilers---notably PCC variants---do not carry strings about within the compiler. Instead, they work as follows: When the lexical analyser sees a double-quote (`"') character, it calls a special routine to collect strings. If the compiler has just begun the initialisation of an array of char, this routine gathers each byte and drops it in the main initialised data space (`.data' or `.data 0'). If not, it switches to the alternate initialised data space (`.data 1') and generates a label, then drops each byte in this space, then resumes the previous space (instruction or main data). This can be seen in the compiler output for, e.g., f() { static char str[] = "main data"; char *p = "alternate data"; } # saw function declaration, so generate prologue for f _f: .word L12 # (end-of-function code moved here by peephole optimiser) subl2 $4,sp .set L12,0x0 # saw `static char str[] = "': generate static local variable `str' .data # main data space L16: # str is L16 .long 0x6e69616d # "main" .long 0x74616420 # " dat" .long 0x61 # "a\0" # end of string # saw `char *p = "': generate anonymous string constant .data 1 # alternate data space L17: # anonymous string is L17 .ascii "alternate data\0" .data # resume previous space .text # finish p = "..." initialisation: moval L17,-4(fp) # p = L17 ret # end of f() This method of handling anonymous aggregates, while expedient (the compiler never carries more than one `thing' in its `head'), has several unpleasant side effects. One is that "text"[0] generates code like cvtbl L17,r11 rather than simply movl 't',r11 Another is that char *p1 = "hello", *p2 = "hello"; generates two separate strings that have the same text, rather than making only one `hello\0' but making p1 and p2 both point to that. Finally, f() { return (sizeof("hello")); } compiles to the following mess: # some junk deleted .data 1 L16: .ascii "hello\0" .text _f: .word 0 movl $6,r0 ret Although the string itself is never used, it is still generated. The latter two problems can be cured without changing the basic anonymous aggregate string builder; the first cannot. -- In-Real-Life: Chris Torek, Univ of MD Computer Science, +1 301 454 7163 Domain: chris@mimsy.umd.edu Path: ...!uunet!mimsy!chris
chris@mimsy.UUCP (Chris Torek) (04/21/88)
In article <2569@umd5.umd.edu> chris@trantor.umd.edu (I, using another machine over the weekend since this one had a bad cache address board) wrote: >[PCC's] method of handling anonymous aggregates, while expedient (the >compiler never carries more than one `thing' in its `head'), has >several unpleasant side effects. ... Another is that > char *p1 = "hello", *p2 = "hello"; >generates two separate strings that have the same text .... [This] can >be cured without changing the basic anonymous aggregate string builder .... Oops. This is almost absurd. One could fix it in /lib/c2, perhaps, or in a separate utility specially constructed for hacking over `.data 1' constructs, but to do it in the compiler requires either backing up the output file, or `carrying the string in its head' (perhaps in a temporary file). -- In-Real-Life: Chris Torek, Univ of MD Comp Sci Dept (+1 301 454 7163) Domain: chris@mimsy.umd.edu Path: uunet!mimsy!chris