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<head>
<base href="https://llvm.org/bugs/" />
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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 --- - Machine code sinking is blocked when SUBREG_TO_REG is seen"
href="https://llvm.org/bugs/show_bug.cgi?id=28852">28852</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Machine code sinking is blocked when SUBREG_TO_REG is seen
</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>All
</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>Common Code Generator Code
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>wmi@google.com
</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>For the testcase 1.c below:
-------------------------------------
unsigned x;
void foo(unsigned long value, unsigned kLengthBits, unsigned* bits, unsigned
long *bit_buffer_64) {
long tmp = kLengthBits * x + 3;
long length = tmp * 5;
int v = 64 - value;
if (__builtin_expect(*bits < (unsigned)(v), 0)) {
*bit_buffer_64 += (value + length);
*bits = length;
}
}
-------------------------------------
length is only used in a cold branch, so it is beneficial to sink the
computation chain targeted to length to the cold branch.
llvm moves "tmp * 5" to the cold branch, but doesn't move "kLengthBits * x +
3".
~/workarea/llvm-r277486/dbuild/bin/clang -O2 -S 1.c
foo: # @foo
.cfi_startproc
# BB#0: # %entry
# kill: %ESI<def> %ESI<kill> %RSI<def>
imull x(%rip), %esi
addl $3, %esi
movl $64, %eax
subl %edi, %eax
cmpl %eax, (%rdx)
jb .LBB0_1
.LBB0_2: # %if.end
retq
.LBB0_1: # %if.then
leaq (%rsi,%rsi,4), %rax
addq %rax, %rdi
addq %rdi, (%rcx)
movl %eax, (%rdx)
jmp .LBB0_2
.Lfunc_end0:
.size foo, .Lfunc_end0-foo
.cfi_endproc
gcc moves all the computations related with length to the cold branch.
~/workarea/gcc-r233722/build/install/bin/gcc -O2 -S 1.c
foo:
.LFB0:
.cfi_startproc
movl $64, %eax
subl %edi, %eax
cmpl (%rdx), %eax
ja .L4
rep ret
.p2align 4,,10
.p2align 3
.L4:
imull x(%rip), %esi
leal 3(%rsi), %eax
leaq (%rax,%rax,4), %rax
leaq (%rdi,%rax), %rsi
addq %rsi, (%rcx)
movl %eax, (%rdx)
ret
.cfi_endproc
llvm doesn't move "kLengthBits * x + 3" because it see a SUBREG_TO_REG which is
generated by a zext for "kLengthBits * x + 3" from i32 to i64.
MachineSinking pass will stop sink instructions once SUBREG_TO_REG is seen.
/// Return true if the instruction should be sunk by MachineSink.
///
/// MachineSink determines on its own whether the instruction is safe to
sink;
/// this gives the target a hook to override the default behavior with
regards
/// to which instructions should be sunk.
/// The default behavior is to not sink insert_subreg, subreg_to_reg, and
/// reg_sequence. These are meant to be close to the source to make it easier
/// to coalesce.
virtual bool shouldSink(const MachineInstr &MI) const {
return !MI.isInsertSubreg() && !MI.isSubregToReg() && !MI.isRegSequence();
}
The check above was added at r114631.
------------------------------------------------------------------------
r114631 | evancheng | 2010-09-22 23:53:00 -0700 (Wed, 22 Sep 2010) | 3 lines
Don't sink insert_subreg, subreg_to_reg, reg_sequence. They are meant to be
close to their sources to facilitate coalescing.
------------------------------------------------------------------------
Considering the check was added before greedy register allocation was
introduced, is it still necessary for coalescing today?</pre>
</div>
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