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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - clang assumes zero-extension of 8-bit arguments in x86, causing interop issues with gcc"
href="https://bugs.llvm.org/show_bug.cgi?id=44228">44228</a>
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<th>Summary</th>
<td>clang assumes zero-extension of 8-bit arguments in x86, causing interop issues with gcc
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<th>Product</th>
<td>clang
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
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<th>OS</th>
<td>Linux
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<th>Status</th>
<td>NEW
</td>
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<th>Severity</th>
<td>enhancement
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<th>Priority</th>
<td>P
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<th>Component</th>
<td>LLVM Codegen
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<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
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<th>Reporter</th>
<td>emilio@crisal.io
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<th>CC</th>
<td>llvm-bugs@lists.llvm.org, neeilans@live.com, richard-llvm@metafoo.co.uk
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<pre>Created <span class=""><a href="attachment.cgi?id=22899" name="attach_22899" title="test-case">attachment 22899</a> <a href="attachment.cgi?id=22899&action=edit" title="test-case">[details]</a></span>
test-case
When receiving 8-bit-wide arguments in extern function, clang seems to assume
the argument has been zero-extended by the caller.
According to <a href="https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92821#c2">https://gcc.gnu.org/bugzilla/show_bug.cgi?id=92821#c2</a>:
<span class="quote">> I believe it is a LLVM bug.
> At least, reading <a href="https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-1.0.pdf">https://github.com/hjl-tools/x86-psABI/wiki/x86-64-psABI-1.0.pdf</a>, I can't find in the Parameter Passing section anything that would say that arguments smaller than 64-bit are passed sign or zero extended to 64-bit like some other psABIs require. The only related thing is
> "When a value of type _Bool is returned or passed in a register or on the stack, bit 0 contains the truth value and bits 1 to 7 shall be zero."
> with a footnote:
> "Other bits are left unspecified, hence the consumer side of those values can rely on it being 0 or 1 when truncated to 8 bit."
> which says that _Bool has only significant low 8 bits and the rest is unspecified.</span >
<a href="https://godbolt.org/z/BNHxEY">https://godbolt.org/z/BNHxEY</a> has a comparison of clang and gcc output for the
attached test-case. GCC correctly does an 8-bit load, disregarding the rest of
the bits in the register.
This causes real problems when gcc-built functions call into llvm-built
functions. See <a href="https://bugzilla.mozilla.org/show_bug.cgi?id=1600735">https://bugzilla.mozilla.org/show_bug.cgi?id=1600735</a> for an
example that happens on Firefox. GCC may not always sign-extend in the caller.
In the Firefox case the LLVM-built function is Rust code, but per the above
godbolt link it also seems to reproduce with C / C++.</pre>
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