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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed optimization: AArch64 backend doesn’t elide function prologue/epilogue emission for noreturn+nounwind+!uwtable functions"
href="https://bugs.llvm.org/show_bug.cgi?id=46667">46667</a>
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<th>Summary</th>
<td>Missed optimization: AArch64 backend doesn’t elide function prologue/epilogue emission for noreturn+nounwind+!uwtable functions
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<th>Product</th>
<td>libraries
</td>
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<th>Version</th>
<td>trunk
</td>
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<th>Hardware</th>
<td>PC
</td>
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<th>OS</th>
<td>All
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<th>Status</th>
<td>NEW
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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>Backend: AArch64
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<th>Assignee</th>
<td>unassignedbugs@nondot.org
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<th>Reporter</th>
<td>vlad@tsyrklevich.net
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<th>CC</th>
<td>arnaud.degrandmaison@arm.com, llvm-bugs@lists.llvm.org, smithp352@googlemail.com, Ties.Stuij@arm.com
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<pre>AArch64FrameLowering doesn’t implement enableCalleeSaveSkip() [1], so it gets
the default base class implementation that always returns false. Simply
implementing this method to return true would slightly slim down
noreturn+nounwind+!uwtable functions.
For example:
$ cat noret.c
int puts(const char *s);
__attribute__((noreturn)) __attribute__((nothrow))
void mytrap(void) {
puts("In mytrap\n");
__builtin_trap();
}
$ clang -arch arm64 -Oz -c -o noret.o noret.c -fomit-frame-pointer
-fno-unwind-tables -msign-return-address=non-leaf
$ objdump -d noret.o
…
0000000000000000 ltmp0:
0: 3f 23 03 d5 paciasp
4: fd 7b bf a9 stp x29, x30, [sp, #-16]!
8: 00 00 00 90 adrp x0, #0
c: 00 00 00 91 add x0, x0, #0
10: 00 00 00 94 bl #0 <ltmp0+0x10>
14: 20 00 20 d4 brk #0x1
If enableCalleeSaveSkip() were to return true on AArch64, the paciasp and stp
would be elided.
[1] <a href="https://reviews.llvm.org/D42898">https://reviews.llvm.org/D42898</a></pre>
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