<html><head><base href="x-msg://242/"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; -webkit-line-break: after-white-space; "><span class="Apple-style-span" style="font-family: Times; "><dt><code>TRUNC_DIV_EXPR</code></dt><dt><code>FLOOR_DIV_EXPR</code></dt><dt><code>CEIL_DIV_EXPR</code></dt><dt><code>ROUND_DIV_EXPR</code></dt><dd>These nodes represent integer division operations that return an integer result. <code>TRUNC_DIV_EXPR</code> rounds towards zero, <code>FLOOR_DIV_EXPR</code>rounds towards negative infinity, <code>CEIL_DIV_EXPR</code> rounds towards positive infinity and <code>ROUND_DIV_EXPR</code> rounds to the closest integer. Integer division in C and C++ is truncating, i.e. <code>TRUNC_DIV_EXPR</code>.<p>The behavior of these operations on signed arithmetic overflow, when dividing the minimum signed integer by minus one, is controlled by the <code>flag_wrapv</code> and <code>flag_trapv</code> variables. <br></p></dd><dt><code>TRUNC_MOD_EXPR</code></dt><dt><code>FLOOR_MOD_EXPR</code></dt><dt><code>CEIL_MOD_EXPR</code></dt><dt><code>ROUND_MOD_EXPR</code></dt><dd>These nodes represent the integer remainder or modulus operation. The integer modulus of two operands <code>a</code> and <code>b</code> is defined as <code>a - (a/b)*b</code> where the division calculated using the corresponding division operator. Hence for <code>TRUNC_MOD_EXPR</code> this definition assumes division using truncation towards zero, i.e. <code>TRUNC_DIV_EXPR</code>. Integer remainder in C and C++ uses truncating division, i.e.<code>TRUNC_MOD_EXPR</code>. </dd></span><div><br></div><div>This is a quote from the gcc internals manual. I've always wondered why cm3cg uses the built-in FLOOR_DIV_EXPR/FLOOR_MOD_EXPR only for known positive operands. What would be wrong about using them also for negative operands?</div><div><br></div><div>
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<br><div><div>On 17 Jan 2010, at 08:55, Jay K wrote:</div><br class="Apple-interchange-newline"><blockquote type="cite"><span class="Apple-style-span" style="border-collapse: separate; font-family: Helvetica; font-size: medium; font-style: normal; font-variant: normal; font-weight: normal; letter-spacing: normal; line-height: normal; orphans: 2; text-indent: 0px; text-transform: none; white-space: normal; widows: 2; word-spacing: 0px; -webkit-border-horizontal-spacing: 0px; -webkit-border-vertical-spacing: 0px; -webkit-text-decorations-in-effect: none; -webkit-text-size-adjust: auto; -webkit-text-stroke-width: 0px; "><div class="hmmessage" style="font-size: 10pt; font-family: Verdana; ">I have to admit I don't fully understand<br>the division and modulo code in hand.c.<br><br><br>I don't understand the old division code,<br>but the documenation and behavior are<br>clear: round down. I understand<br>why my version works, though it might<br>depend on non-guaranteed behavior in C division<br>for the same sign cases. An earlier version<br>was clearer since it avoided doing anything<br>with negative numbers execpt negating them<br>(and carefully at that).<br><br><br>Modulo is much less clear to me.<br>From testing vs. the documentation, I know the<br>old code was wrong, though close.<br>I made some guesses based on the old code and<br>ran a variety of inputs through it.<br>My version matches the old version, except<br>where the old version is wrong.<br>I could write a clearly correct version, but it<br>would be perhaps much less efficient, just<br>computing a - b * div(a, b).<br><br><br>I tried running all 32bit inputs through some of it but it was<br>going to take too long.<br>I could maybe leave that running a few days if needed.<br><br> - Jay<br><br></div></span></blockquote></div><br></body></html>