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<base href="https://bugs.llvm.org/">
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<th>Bug ID</th>
<td><a class="bz_bug_link
bz_status_NEW "
title="NEW - MSVC rotate intrinsics don't (just) generate rotates on x86-64"
href="https://bugs.llvm.org/show_bug.cgi?id=37387">37387</a>
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<th>Summary</th>
<td>MSVC rotate intrinsics don't (just) generate rotates on x86-64
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</td>
</tr>
<tr>
<th>Version</th>
<td>6.0
</td>
</tr>
<tr>
<th>Hardware</th>
<td>PC
</td>
</tr>
<tr>
<th>OS</th>
<td>Windows NT
</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>
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<th>Component</th>
<td>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>fabiang@radgametools.com
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>This simple test:
// ---- begin
#include <intrin.h>
extern "C" unsigned long long f(unsigned long long a, int b)
{
return _rotl64(a, b);
}
extern "C" unsigned long long g(unsigned long long a, int b)
{
return (a << (b & 63)) | (a >> (-b & 63));
}
// ---- end
produces (on x86-64 using clang 6.0 release; only quoting the relevant bits):
# ---- begin
f: # @f
# %bb.0:
movq %rcx, %r8
andl $63, %edx
movq %r8, %rax
movl %edx, %ecx
rolq %cl, %rax
testl %edx, %edx
cmoveq %r8, %rax
retq
g: # @g
# %bb.0:
movq %rcx, %rax
movl %edx, %ecx
rolq %cl, %rax
retq
# ---- end
The corresponding IR is:
; ---- begin
; Function Attrs: norecurse nounwind readnone sspstrong uwtable
define i64 @f(i64, i32) local_unnamed_addr #0 {
%3 = and i32 %1, 63
%4 = zext i32 %3 to i64
%5 = sub nsw i64 64, %4
%6 = shl i64 %0, %4
%7 = lshr i64 %0, %5
%8 = or i64 %7, %6
%9 = icmp eq i32 %3, 0
%10 = select i1 %9, i64 %0, i64 %8
ret i64 %10
}
; Function Attrs: norecurse nounwind readnone sspstrong uwtable
define i64 @g(i64, i32) local_unnamed_addr #0 {
%3 = and i32 %1, 63
%4 = zext i32 %3 to i64
%5 = shl i64 %0, %4
%6 = sub nsw i32 0, %1
%7 = and i32 %6, 63
%8 = zext i32 %7 to i64
%9 = lshr i64 %0, %8
%10 = or i64 %5, %9
ret i64 %10
}
; ---- end
The problem is the expansion chosen for the rotr/rotl intrinsics in
CGBuiltin.cpp CodeGenFunction::EmitBuiltinExpr, presumably to avoid
implementation-specific behavior from the right shift by 64-b.
Note that the alternative expansion for rotate-left given in the code for g
avoids the problematic select, is well-defined, and already gets matched to ROL
(in the x86-64 backend anyway), so it seems like a good alternative.</pre>
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