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Path: utzoo!mnetor!seismo!caip!clyde!watmath!rbutterworth
From: rbutterworth@watmath.UUCP (Ray Butterworth)
Newsgroups: net.lang.c
Subject: Bug in 4.2 cc code generator (result type of assignment)
Message-ID: <3150@watmath.UUCP>
Date: Sun, 14-Sep-86 11:59:11 EDT
Article-I.D.: watmath.3150
Posted: Sun Sep 14 11:59:11 1986
Date-Received: Sun, 14-Sep-86 20:21:52 EDT
Distribution: net
Organization: U of Waterloo, Ontario
Lines: 64

/*
 * The following two loops should be identical,
 * the second test possibly generating more efficient code.
 * i.e. ((c=exp1),(c!=exp2))  versus  ((c=exp1)!=(exp2)).

 * This is true on all the other compilers I have tried,
 * regardless of sign-extension, byte order, word size, etc.

 * But on the 4.2 cc, the second test rather than being more
 * efficient, actually generates more code, and in fact generates
 * incorrect code (the loop never terminates (well almost never)).
 * In this particular case it seems to think that the type of
 * (c=exp1) is the same as the type of (exp1) and not that of (c).
 * According to both K&R and the proposed X3J11 this is wrong.

 * (I realize that the "&0377" is actually redundant, but it is
 * one way of invoking the bug in this contrived example.)
 */

main()
{
    char *data="see should_not_see";
    char stop='\345';
    char *p;
    char c;

    data[3]=stop;

    p = data;
    printf("Using comma operator\n");
    while ( (c=(*p++&0377)), (c!=stop) )
        printf("%3.3o ", c);
    printf("\n");

    p = data;
    printf("Using asignment!=\n");
    while ( (c=(*p++&0377))!=stop )
        printf("%3.3o ", c);
    printf("\n");

    return 0;
}

/*
 *This is the correct code generated for the first loop:
L19:
    cvtbl   *-12(fp),r0
    bicl2   $-256,r0
    cvtlb   r0,-13(fp)
    incl    -12(fp)
    cmpb    -13(fp),-5(fp)
    jeql    L20

 *This is the incorrect code generated for the second (more efficient) loop:
L24:
    movl    -12(fp),r0
    incl    -12(fp)
    cvtbl   (r0),r0
    bicl2   $-256,r0
    cvtlb   r0,-13(fp)
    cvtbl   -5(fp),r1
    cmpl    r0,r1
    jeql    L25
 */