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<base href="https://bugs.llvm.org/">
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<body><table border="1" cellspacing="0" cellpadding="8">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - Induction on fmuladd cannot be vectorized"
href="https://bugs.llvm.org/show_bug.cgi?id=33338">33338</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>Induction on fmuladd cannot be vectorized
</td>
</tr>
<tr>
<th>Product</th>
<td>libraries
</td>
</tr>
<tr>
<th>Version</th>
<td>4.0
</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>Loop Optimizer
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>yyc1992@gmail.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>IIUC it should always be valid (though not necessarily performable) to replace
`llvm.fmuladd` with `fmul` and `fadd` so it is a little surprising to see that
`llvm.fmuladd` can cause worse vectorization when it's used on induction
variable.
Running through `opt -O2` the following code does not vectorize but it
vectorizes if the `fmuladd` is replaced with `fmul` and `fadd` as shown in the
commented out code.
```
target datalayout = "e-m:e-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-unknown-linux-gnu"
; Function Attrs: nounwind readonly uwtable
define double @f(double* nocapture readonly, double* nocapture readonly, i32)
local_unnamed_addr #0 {
%4 = icmp sgt i32 %2, 0
br i1 %4, label %5, label %8
; <label>:5: ; preds = %3
%6 = zext i32 %2 to i64
br label %10
; <label>:7: ; preds = %10
br label %8
; <label>:8: ; preds = %7, %3
%9 = phi double [ 0.000000e+00, %3 ], [ %17, %7 ]
ret double %9
; <label>:10: ; preds = %5, %10
%11 = phi i64 [ %18, %10 ], [ 0, %5 ]
%12 = phi double [ %17, %10 ], [ 0.000000e+00, %5 ]
%13 = getelementptr inbounds double, double* %0, i64 %11
%14 = load double, double* %13, align 8
%15 = getelementptr inbounds double, double* %1, i64 %11
%16 = load double, double* %15, align 8
%17 = tail call fast double @llvm.fmuladd.f64(double %14, double %16, double
%12)
; %r = fmul fast double %14, %16
; %17 = fadd fast double %r, %12
%18 = add nuw nsw i64 %11, 1
%19 = icmp eq i64 %18, %6
br i1 %19, label %7, label %10
}
; Function Attrs: nounwind readnone
declare double @llvm.fmuladd.f64(double, double, double) #1
attributes #0 = { nounwind readonly uwtable "target-features"="+fma" }
attributes #1 = { nounwind readnone }
```</pre>
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