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<p class="MsoNormal">MC currently parses assembly floats in parseExpression by extracting the string for the Real token and parsing this to a double precision floating point value; this value is then bitcast into an integer and an MCExprConstant is created
from this. This has some issues:<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>It isn’t possible to determine after parseExpression whether a integer or a floating point value was parsed<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>Instructions which expect an integer and use parse expression allow strange operands as the value becomes bitcast to an integer, for example:<o:p></o:p></p>
<p class="MsoListParagraph">add r0, r0, lsl #<span style="font-family:"Courier New"">4.94065645841246544176568792868E-323<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="font-family:"Courier New"">will assemble with llvm-mc –triple armv8<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="font-family:"Courier New"">as it is read as<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="font-family:"Courier New"">add r0, r0, lsl #10<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>Expressions allow floats and expressions are attempted to be evaluated , any float in an expression gets represented by its integer representation and this gets used when evaluating an expressions<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I attach a patch which is an attempt at fixing the above issues by introducing an MCFloatExpr which stores the float value as an APFloat and also by adding an APFloat to MCValue to allow float assembler immediates, this solves the above
by:<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>You can determine the type parsed bases on the MCExpr returned<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>As most places already check that value returned is MCConstantExpr it means that the MCFloatExprs are only permitted where they have been added in this patch<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo1">
<![if !supportLists]><span style="font-family:"Courier New""><span style="mso-list:Ignore">o<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>This prevents the above instruction from being assembled an instead throws the following error:
<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:72.0pt">“error: invalid immediate shift value”<o:p></o:p></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo1"><![if !supportLists]><span style="font-family:Symbol"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>The evaluation functions now take an MCValue which can represent either a float or an integer, and the evaluation proceeds based on the type of the value so that float expressions treat the values as floats<o:p></o:p></p>
<p class="MsoListParagraph" style="margin-left:72.0pt;text-indent:-18.0pt;mso-list:l0 level2 lfo1">
<![if !supportLists]><span style="font-family:"Courier New""><span style="mso-list:Ignore">o<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]>When the types are mixed I convert the integers to floats and continue, so the overall result will be a float so floats can only be used when expected<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:54.0pt"><o:p> </o:p></p>
<p class="MsoNormal">I would like some feedback on interest in this patch and changing the way floats are handled and any suggestions and criticisms and whether this is a good or bad way of approaching this.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Thanks,<o:p></o:p></p>
<p class="MsoNormal">Luke Cheeseman<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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