alpha/72024: LONG_MIN / 1 sends a "Floating exception"

[Peter Edwards]

The following reply was made to PR alpha/72024; it has been noted by GNATS.

From: Peter Edwards <peadar at freebsd.org>
To: freebsd-gnats-submit at FreeBSD.org, ppelissi at caramail.com
Cc:  
Subject: Re: alpha/72024: LONG_MIN / 1 sends a "Floating exception"
Date: Sun, 10 Oct 2004 00:03:13 +0100

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 Ooh a good excuse to look at Alpha assembler. (/me googles for the alpha 
 instruction set.)
 
 (Can someone on the -alpha list tell me if this reasoning is correct, 
 and if I should commit the patch?)
 
 Alpha appears not to have an integer division instruction, so this is 
 implemented by divq(), the source of which is generated from 
 "src/lib/libc/alpha/gen/divrem4"
 
 (For reading the macros when preprocessing as divq, S is true, OP is 
 "div", and WORDSIZE is 64)
 
 If I'm reading it correctly, the start of this function decides what 
 sign the result should have, then gets the absolute value of each 
 operand, (search for "subq zero, A, A" and "subq zero, B, B".) Note, for 
 "LONG_MIN", or 0x8000000000000000, 2's compliment subtraction for zero 
 gives back the 0x8000000000000000
 
 The body of the work is done with logical shifts, so everything still 
 works when treating the dividend as an unsigned long.
 
 At the end of the function, the code takes the calculated result, and 
 decides if it needs to negate it:
 
 >         /* Check to see if we should negate it. */
 >         subqv   zero, RESULT, T_0
 >         cmovlbs NEG, T_0, RESULT
 
 
 The "v" here means "trap overflows", which is why we blow up. LONG_MIN 
 is the only value that _can_ blow up here, and you can only reach this 
 case if you start with LONG_MIN / 1, if my reasoning isn't flawed.
 So, I think it's perfectly safe to remove the "v" from the subq 
 instruction. i.e., I think the attached patch is correct.
 
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 Index: lib/libc/alpha/gen/divrem.m4
 ===================================================================
 RCS file: /usr/cvs/FreeBSD-CVS/src/lib/libc/alpha/gen/divrem.m4,v
 retrieving revision 1.3
 diff -u -r1.3 divrem.m4
 --- lib/libc/alpha/gen/divrem.m4	10 May 2000 19:04:57 -0000	1.3
 +++ lib/libc/alpha/gen/divrem.m4	9 Oct 2004 22:54:36 -0000
 @@ -172,7 +172,7 @@
  ifelse(S, `true',
  `
  	/* Check to see if we should negate it. */
 -	subqv	zero, RESULT, T_0
 +	subq	zero, RESULT, T_0
  	cmovlbs	NEG, T_0, RESULT
  ')
  
 
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