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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 --- - [PPC] Constant loads not hoisted out of loop"
href="https://llvm.org/bugs/show_bug.cgi?id=30729">30729</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>[PPC] Constant loads not hoisted out of loop
</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>Scalar Optimizations
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>echristo@gmail.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>In f(), LLVM hoists the loads for constants k1 and k2 outside loop. In both
g() and h(), the loads stay inside, resulting in a longer loop body. Both g()
and h() have an if-statement enclosing the loads, so always loading the
constants is not necessarily better, since it is possible that the if-condition
may not be true. However, when adding __builtin_expect in h() to hint the
compiler that the loads are very likely to be executed. In that case, I'd
probably expect the compiler to move the loads out of the loop body.
---
#include <altivec.h>
#include <stdint.h>
struct S {
vector int k1;
vector int k2;
vector int f(uint64_t n, vector int x) const;
vector int g(uint64_t n, vector int x) const;
vector int h(uint64_t n, vector int x) const;
};
vector int S::f(uint64_t n, vector int x) const {
const uint64_t base = 16;
for ( ; n ; n /= base) {
x += vec_ld(0, &k1);
x ^= vec_ld(0, &k2);
}
return x;
}
vector int S::g(uint64_t n, vector int x) const {
const uint64_t base = 16;
for ( ; n ; n /= base) {
const uint64_t d = n % base;
if (d != 0) {
x += vec_ld(0, &k1);
x ^= vec_ld(0, &k2);
}
}
return x;
}
vector int S::h(uint64_t n, vector int x) const {
const uint64_t base = 16;
for ( ; n ; n /= base) {
const uint64_t d = n % base;
if (__builtin_expect(d != 0, 1)) {
x += vec_ld(0, &k1);
x ^= vec_ld(0, &k2);
}
}
return x;
}
---
.text
.abiversion 2
.file "constants.cc"
.globl _ZNK1S1fEyDv4_i
.p2align 4
.type _ZNK1S1fEyDv4_i,@function
_ZNK1S1fEyDv4_i: # @_ZNK1S1fEyDv4_i
.Lfunc_begin0:
# BB#0:
cmpldi 4, 0
beq 0, .LBB0_4
# BB#1:
li 5, 16
lvx 3, 0, 3
xxlor 34, 34, 34
lvx 4, 3, 5
.p2align 4
.LBB0_2: # =>This Inner Loop Header: Depth=1
vadduwm 2, 3, 2
rldicl. 4, 4, 60, 4
xxlxor 34, 34, 36
bne 0, .LBB0_2
# BB#3:
# kill: %V2<def> %V2<kill> %VSH2<kill>
blr
.LBB0_4:
xxlor 34, 34, 34
# kill: %V2<def> %V2<kill> %VSH2<kill>
blr
.long 0
.quad 0
.Lfunc_end0:
.size _ZNK1S1fEyDv4_i, .Lfunc_end0-.Lfunc_begin0
.globl _ZNK1S1gEyDv4_i
.p2align 4
.type _ZNK1S1gEyDv4_i,@function
_ZNK1S1gEyDv4_i: # @_ZNK1S1gEyDv4_i
.Lfunc_begin1:
# BB#0:
cmpldi 4, 0
beq 0, .LBB1_6
# BB#1:
xxlor 34, 34, 34
addi 5, 3, 16
.p2align 5
.LBB1_2: # =>This Inner Loop Header: Depth=1
rldicl. 6, 4, 0, 60
beq 0, .LBB1_4
# BB#3: # in Loop: Header=BB1_2 Depth=1
lvx 3, 0, 3
lvx 4, 0, 5
vadduwm 2, 3, 2
xxlxor 34, 34, 36
.LBB1_4: # in Loop: Header=BB1_2 Depth=1
rldicl. 4, 4, 60, 4
bne 0, .LBB1_2
# BB#5:
# kill: %V2<def> %V2<kill> %VSH2<kill>
blr
.LBB1_6:
xxlor 34, 34, 34
# kill: %V2<def> %V2<kill> %VSH2<kill>
blr
.long 0
.quad 0
.Lfunc_end1:
.size _ZNK1S1gEyDv4_i, .Lfunc_end1-.Lfunc_begin1
.globl _ZNK1S1hEyDv4_i
.p2align 4
.type _ZNK1S1hEyDv4_i,@function
_ZNK1S1hEyDv4_i: # @_ZNK1S1hEyDv4_i
.Lfunc_begin2:
# BB#0:
cmpldi 4, 0
beq 0, .LBB2_6
# BB#1:
xxlor 34, 34, 34
addi 5, 3, 16
.p2align 5
.LBB2_2: # =>This Inner Loop Header: Depth=1
rldicl. 6, 4, 0, 60
beq 0, .LBB2_4
# BB#3: # in Loop: Header=BB2_2 Depth=1
lvx 3, 0, 3
lvx 4, 0, 5
vadduwm 2, 3, 2
xxlxor 34, 34, 36
.LBB2_4: # in Loop: Header=BB2_2 Depth=1
rldicl. 4, 4, 60, 4
bne 0, .LBB2_2
# BB#5:
# kill: %V2<def> %V2<kill> %VSH2<kill>
blr
.LBB2_6:
xxlor 34, 34, 34
# kill: %V2<def> %V2<kill> %VSH2<kill>
blr
.long 0
.quad 0
.Lfunc_end2:
.size _ZNK1S1hEyDv4_i, .Lfunc_end2-.Lfunc_begin2</pre>
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