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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Missed switch-to-lookup-table transformation for switch with "holes""
href="https://bugs.llvm.org/show_bug.cgi?id=50405">50405</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Missed switch-to-lookup-table transformation for switch with "holes"
</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>enhancement
</td>
</tr>
<tr>
<th>Priority</th>
<td>P
</td>
</tr>
<tr>
<th>Component</th>
<td>Scalar Optimizations
</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></table>
<p>
<div>
<pre>Consider the code below, which is written in a way that should be perfect for
transforming to a lookup table:
$ cat /tmp/a.c
const char *g();
const char *f(int x) {
char *ret;
switch (x) {
case 0: ret = "0"; break;
case 1: ret = "1"; break;
case 3: ret = "3"; break;
case 5: ret = "5"; break;
case 6: ret = "6"; break;
case 8: ret = "8"; break;
case 10: ret = "10"; break;
case 11: ret = "11"; break;
case 12: ret = "12"; break;
case 15: ret = "15"; break;
case 17: ret = "17"; break;
case 19: ret = "19"; break;
case 22: ret = "22"; break;
case 24: ret = "24"; break;
case 27: ret = "27"; break;
case 29: ret = "29"; break;
case 31: ret = "31"; break;
case 32: ret = "32"; break;
case 33: ret = "33"; break;
case 35: ret = "35"; break;
default: ret = 0;
}
if (!ret) { return g(); }
return ret;
}
However, LLVM will lower the switch with a jump table:
$ bin/clang -target i686-unknown-linux -O2 -S -o - /tmp/a.c
.text
.file "a.c"
.globl f # -- Begin function f
.p2align 4, 0x90
.type f,@function
f: # @f
# %bb.0: # %entry
movl 4(%esp), %ecx
cmpl $35, %ecx
ja .LBB0_20
# %bb.1: # %entry
movl $.L.str, %eax
jmpl *.LJTI0_0(,%ecx,4)
.LBB0_21: # %sw.bb1
movl $.L.str.1, %eax
.LBB0_22: # %cleanup
retl
.LBB0_20: # %if.then
jmp g # TAILCALL
.LBB0_2: # %sw.bb2
movl $.L.str.2, %eax
retl
.LBB0_3: # %sw.bb3
movl $.L.str.3, %eax
retl
.LBB0_4: # %sw.bb4
movl $.L.str.4, %eax
retl
[...]
If the holes are plugged in the switch, the generated code is the expected:
$ cat /tmp/a.c
const char *g();
const char *f(int x) {
char *ret;
switch (x) {
case 0: ret = "0"; break;
case 1: ret = "1"; break;
case 2: ret = "1"; break;
case 3: ret = "3"; break;
case 4: ret = "3"; break;
case 5: ret = "5"; break;
case 6: ret = "6"; break;
case 7: ret = "6"; break;
case 8: ret = "8"; break;
case 9: ret = "8"; break;
case 10: ret = "10"; break;
case 11: ret = "11"; break;
case 12: ret = "12"; break;
case 13: ret = "12"; break;
case 14: ret = "12"; break;
case 15: ret = "15"; break;
case 16: ret = "15"; break;
case 17: ret = "17"; break;
case 18: ret = "17"; break;
case 19: ret = "19"; break;
case 20: ret = "19"; break;
case 21: ret = "19"; break;
case 22: ret = "22"; break;
case 23: ret = "22"; break;
case 24: ret = "24"; break;
case 25: ret = "24"; break;
case 26: ret = "24"; break;
case 27: ret = "27"; break;
case 28: ret = "27"; break;
case 29: ret = "29"; break;
case 30: ret = "29"; break;
case 31: ret = "31"; break;
case 32: ret = "32"; break;
case 33: ret = "33"; break;
case 34: ret = "33"; break;
case 35: ret = "35"; break;
default: ret = 0;
}
if (!ret) { return g(); }
return ret;
}
$ bin/clang -target i686-unknown-linux -O2 -S -o - /tmp/a.c
.text
.file "a.c"
.globl f # -- Begin function f
.p2align 4, 0x90
.type f,@function
f: # @f
# %bb.0: # %entry
movl 4(%esp), %eax
cmpl $36, %eax
jae .LBB0_2
# %bb.1: # %switch.lookup
movl .Lswitch.table.f(,%eax,4), %eax
retl
.LBB0_2: # %if.then
jmp g # TAILCALL
.Lfunc_end0:
.size f, .Lfunc_end0-f
# -- End function
[...]</pre>
</div>
</p>
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