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<base href="https://bugs.llvm.org/">
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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 - 32 x 32 but "multhi" generates horible code for MIPS"
href="https://bugs.llvm.org/show_bug.cgi?id=36884">36884</a>
</td>
</tr>
<tr>
<th>Summary</th>
<td>32 x 32 but "multhi" generates horible code for MIPS
</td>
</tr>
<tr>
<th>Product</th>
<td>clang
</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>LLVM Codegen
</td>
</tr>
<tr>
<th>Assignee</th>
<td>unassignedclangbugs@nondot.org
</td>
</tr>
<tr>
<th>Reporter</th>
<td>npl@chello.at
</td>
</tr>
<tr>
<th>CC</th>
<td>llvm-bugs@lists.llvm.org
</td>
</tr></table>
<p>
<div>
<pre>Following code was compiled with "-O2 -target mips", also see the compiler
explorer: <a href="https://godbolt.org/g/t7C267">https://godbolt.org/g/t7C267</a>
Usecase was generating efficient C code for division by 100 with suitable small
inputs. The explicit variant is a tiny bit more efficient for most compilers,
but it totally explodes on MIPS + clang
------ Source -----------------
#include <stdint.h>
uint32_t div100x(uint32_t uintVal, uint32_t *r)
{
const uint32_t divrem[] = {
uintVal / 100U,
uintVal % 100U
};
*r = divrem[1];
return divrem[0];
}
uint32_t div100y(uint32_t uintVal, uint32_t *r)
{
const uint32_t divrem[] = {
(uint32_t)(((uint64_t)uintVal * 42949673U) >> 32),
uintVal - (uint32_t)(((uint64_t)uintVal * 42949673U) >> 32) * 100U
};
*r = divrem[1];
return divrem[0];
}
------- Generated code -------------------
div100y(unsigned int, unsigned int*): # @div100y(unsigned int, unsigned int*)
addiu $sp, $sp, -8
sw $fp, 4($sp) # 4-byte Folded Spill
move $fp, $sp
sll $1, $4, 3
addu $2, $1, $4
sll $3, $4, 5
addu $6, $3, $2
sltu $3, $6, $3
sltu $1, $2, $1
srl $2, $4, 29
addu $1, $2, $1
srl $2, $4, 27
addu $1, $2, $1
sll $2, $4, 10
addu $1, $1, $3
subu $3, $2, $6
sll $7, $4, 13
addu $3, $7, $3
sll $8, $4, 15
addu $9, $8, $3
sll $10, $4, 20
srl $11, $4, 22
subu $1, $11, $1
sltu $2, $2, $6
subu $6, $10, $9
srl $11, $4, 7
sll $12, $4, 25
srl $13, $4, 9
sll $14, $4, 23
srl $15, $4, 12
srl $24, $4, 17
srl $25, $4, 19
subu $1, $1, $2
addu $1, $25, $1
sltu $2, $3, $7
addu $1, $1, $2
addu $1, $24, $1
sltu $2, $9, $8
addu $1, $1, $2
subu $1, $15, $1
sltu $2, $10, $9
subu $1, $1, $2
addu $2, $14, $6
addu $1, $13, $1
addu $3, $12, $2
sltu $2, $2, $14
addu $1, $1, $2
sltu $2, $3, $12
addu $1, $11, $1
addu $2, $1, $2
sll $1, $2, 2
sll $3, $2, 5
subu $1, $3, $1
sll $3, $2, 7
subu $1, $3, $1
subu $1, $4, $1
sw $1, 0($5)
move $sp, $fp
lw $fp, 4($sp) # 4-byte Folded Reload
jr $ra
addiu $sp, $sp, 8</pre>
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