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<base href="https://llvm.org/bugs/" />
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW --- - Inefficient passing of byval parameters"
href="https://llvm.org/bugs/show_bug.cgi?id=27522">27522</a>
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</tr>
<tr>
<th>Summary</th>
<td>Inefficient passing of byval parameters
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>trunk
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Linux
</td>
</tr>
<tr>
<th>Status</th>
<td>NEW
</td>
</tr>
<tr>
<th>Severity</th>
<td>normal
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Backend: X86
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>hans@chromium.org
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr>
<tr>
<th>Classification</th>
<td>Unclassified
</td>
</tr></table>
<p>
<div>
<pre>Consider:
target datalayout = "e-m:x-p:32:32-i64:64-f80:32-n8:16:32-a:0:32-S32"
target triple = "i686-pc-windows-msvc18.0.0"
%struct.S = type { i32 }
define void @f() {
entry:
%agg.tmp = alloca %struct.S
%x = getelementptr inbounds %struct.S, %struct.S* %agg.tmp, i32 0, i32 0
store i32 42, i32* %x
call void @g(%struct.S* byval nonnull align 4 %agg.tmp)
ret void
}
declare void @g(%struct.S* byval align 4)
$ bin/llc /tmp/a.ll -o -
[..]
_f: # @f
# BB#0: # %entry
subl $8, %esp
movl $42, 4(%esp)
movl 4(%esp), %eax
movl %eax, (%esp)
calll _g
addl $8, %esp
retl
That's quite a round-about way to put 42 on the stack.
Another example:
struct S {
int x;
};
void g(S);
void f(int x) {
g({x});
}
$ bin/clang++ -target i686-pc-win32 -O3 -S -o - /tmp/a.cc
[..]
# BB#0: # %entry
subl $8, %esp
movl 12(%esp), %eax
movl %eax, 4(%esp)
movl 4(%esp), %eax
movl %eax, (%esp)
calll "?g@@YAXUS@@@Z"
addl $8, %esp
retl
We could just jump to g() directly instead :-)</pre>
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