73 lines
6.0 KiB
XML
73 lines
6.0 KiB
XML
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<title id="nj-blog-overlap">Writing an 18-byte string with two overlapping 16-byte loads and stores</title>
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<desc id="nj-blog-overlap-d">The 18 bytes of profile_link_color. Load a covers bytes 0 to 15, load b covers bytes 2 to 17, so bytes 2 to 15 are in both. Neither contains a byte that needs escaping, so a is stored at the destination and b 2 bytes later; the second store writes bytes 2 to 15 again with the same values. This works for any length from 16 to 32 bytes without a loop.</desc>
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<text class="fg-h" x="40" y="20">a string of n = 18 bytes: too long for one 16-byte vector, too short for a loop</text>
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<text class="fg-code" x="363.5" y="58" text-anchor="middle">n</text>
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<rect class="nj-charts-c3" fill-opacity=".3" x="381" y="34" width="27" height="36" rx="4"/>
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<text class="fg-code" x="394.5" y="58" text-anchor="middle">k</text>
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<text class="fg-code" x="518.5" y="58" text-anchor="middle">l</text>
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<text class="nj-charts-tick" x="53.5" y="86">0</text>
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<text class="nj-charts-tick" x="115.5" y="86">2</text>
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<text class="nj-charts-tick" x="518.5" y="86">15</text>
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<text class="nj-charts-tick" x="580.5" y="86">17</text>
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<text class="fg-code" x="286" y="116" text-anchor="middle" style="font-size:14px">a = load 16 bytes at p</text>
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<text class="fg-code" x="348" y="146" text-anchor="middle" style="font-size:14px">b = load 16 bytes at p + n − 16</text>
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<text class="fg-note fg-note--l" x="40" y="182">Both loads lie inside the string, and together they cover all of it; bytes 2 to 15 are in both.</text>
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<text class="fg-note fg-note--l" x="40" y="204">Test a and b for a quote, a backslash, a control character or 0x7F: none, so write</text>
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<text class="fg-code" x="40" y="234">store a at d, store b at d + n − 16</text>
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<text class="fg-note fg-note--l" x="40" y="262">The second store writes bytes 2 to 15 again, with the same values. For any n from 16 to 32:</text>
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<text class="fg-note fg-note--l" x="40" y="280">two loads, one test, two stores, no loop and no branch on the length.</text>
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