<html>
<head>
<base href="https://bugs.llvm.org/">
</head>
<body><table border="1" cellspacing="0" cellpadding="8">
<tr>
<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Suoptimal multiplications with -Ofast"
href="https://bugs.llvm.org/show_bug.cgi?id=42681">42681</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Suoptimal multiplications with -Ofast
</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>david.bolvansky@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>#include <math.h>
double foo(double x, unsigned int n) {
double result = 0.0;
unsigned int i = 1;
double numerator = 1.0;
double denumerator = 1.0;
x = 1.0 - x;
while (i <= n) {
numerator = numerator * ((x - 1) / x);
result -= numerator / denumerator;
denumerator++;
i++;
}
return result;
}
Clang-8 -Ofast
foo(double, unsigned int): # @foo(double,
unsigned int)
test edi, edi
je .LBB0_1
movsd xmm2, qword ptr [rip + .LCPI0_0] # xmm2 = mem[0],zero
movapd xmm1, xmm0
subsd xmm1, xmm2
divsd xmm0, xmm1
xorpd xmm1, xmm1
mov eax, 1
movapd xmm3, xmm2
movapd xmm4, xmm2
.LBB0_3: # =>This Inner Loop Header: Depth=1
mulsd xmm4, xmm0
movapd xmm5, xmm4
divsd xmm5, xmm3
subsd xmm1, xmm5
addsd xmm3, xmm2
add eax, 1
cmp eax, edi
jbe .LBB0_3
movapd xmm0, xmm1
ret
.LBB0_1:
xorps xmm1, xmm1
movaps xmm0, xmm1
ret
Clang 9/10 -Ofast
foo(double, unsigned int): # @foo(double,
unsigned int)
test edi, edi
je .LBB0_1
movsd xmm2, qword ptr [rip + .LCPI0_0] # xmm2 = mem[0],zero
movapd xmm1, xmm2
subsd xmm1, xmm0
movapd xmm3, xmm2
divsd xmm3, xmm1
xorpd xmm1, xmm1
mov eax, 1
movapd xmm4, xmmword ptr [rip + .LCPI0_1] # xmm4 = [-0.0E+0,-0.0E+0]
movapd xmm5, xmm2
movapd xmm6, xmm2
.LBB0_3: # =>This Inner Loop Header: Depth=1
mulsd xmm6, xmm0
mulsd xmm6, xmm3
movapd xmm7, xmm6
xorpd xmm7, xmm4
divsd xmm6, xmm5
addsd xmm1, xmm6
addsd xmm5, xmm2
add eax, 1
movapd xmm6, xmm7
cmp eax, edi
jbe .LBB0_3
movapd xmm0, xmm1
ret
.LBB0_1:
xorps xmm1, xmm1
movaps xmm0, xmm1
ret
The loop body with new Clang looks worse .. 2x mulsd, 2x addsd..
<a href="https://godbolt.org/z/zZRxbE">https://godbolt.org/z/zZRxbE</a>
Clang 8
%12 = fmul fast double %10, %5, !dbg !392
%13 = fdiv fast double %12, %9, !dbg !394
%14 = fsub fast double %8, %13, !dbg !395
Clang 9
%12 = fmul fast double %10, %0, !dbg !384
%13 = fmul fast double %12, %6
%14 = fsub fast double -0.000000e+00, %13
%15 = fdiv fast double %14, %9, !dbg !386
%16 = fsub fast double %8, %15, !dbg !387
So some pattern probably pessimized this code..</pre>
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