<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=UTF-8">
</head>
<body>
<p>If done in the assembler, this is branch relaxation. You need to
implement the calls backs mayNeedRelaxation, and relaxInstruction
on your target MCAsmBackend. The iteration relaxation logic
already exists, see the generic MC code. <br>
</p>
<p>Philip<br>
</p>
<div class="moz-cite-prefix">On 10/5/20 8:20 PM, Kai Wang via
llvm-dev wrote:<br>
</div>
<blockquote type="cite"
cite="mid:CAHfQQ_U_rYPnyZKUWxOPb9Kf7jpZMPbt6OpHgPugPBDE5O1rmQ@mail.gmail.com">
<meta http-equiv="content-type" content="text/html; charset=UTF-8">
<div dir="ltr">
<div>Correct the title.</div>
<br>
<div class="gmail_quote">
<div dir="ltr" class="gmail_attr">On Tue, Oct 6, 2020 at 11:11
AM Kai Wang <<a href="mailto:kai.wang@sifive.com"
moz-do-not-send="true">kai.wang@sifive.com</a>> wrote:<br>
</div>
<blockquote class="gmail_quote" style="margin:0px 0px 0px
0.8ex;border-left:1px solid
rgb(204,204,204);padding-left:1ex">
<div dir="ltr">
<div>Hi all,</div>
<div><br>
</div>
In RISC-V ISA, the range of conditional branches is within
4KiB. In current implementation, if the branch target is
out of range, LLVM MC will issue an error message to tell
users it could not resolve the fixup record. I have
compared the result with the GNU assembler. GNU assembler
will convert the branch to inverted one plus jump to make
the branch possible. The range of unconditional jump is
1MiB. It looks like<br>
<br>
##########################<br>
bne a0, a1, FAR_BRANCH<br>
…<br>
FAR_BRANCH:<br>
<br>
converted to
<div><br>
##########################<br>
beq a0, a1, SKIP_J<br>
j FAR_BRANCH<br>
SKIP_J:<br>
…<br>
FAR_BRANCH:<br>
<br>
I found there is a target hook, relaxInstruction, that
tries to achieve the similar goal. However, the target
hook only replaces one MCInst with another one with a
larger branch range. For example, c.beqz will be
converted to beq in the RISC-V backend if the fixup
value is out of range. There seems no target hook to
convert one MCInst to a complex pattern in LLVM MC. Do I
miss something obvious?<br>
<br>
I found there is a target hook, finishLayout, to
manipulate the code generated. Does it make sense to
implement the feature in finishLayout? Or is there any
better idea to achieve the conversion? Thanks a lot.<br>
<div><br>
</div>
<div>Best,</div>
<div>Kai</div>
</div>
</div>
</blockquote>
</div>
</div>
<br>
<fieldset class="mimeAttachmentHeader"></fieldset>
<pre class="moz-quote-pre" wrap="">_______________________________________________
LLVM Developers mailing list
<a class="moz-txt-link-abbreviated" href="mailto:llvm-dev@lists.llvm.org">llvm-dev@lists.llvm.org</a>
<a class="moz-txt-link-freetext" href="https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev">https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev</a>
</pre>
</blockquote>
</body>
</html>