| Issue |
144422
|
| Summary |
[RISCV] failure to use mulw for 32 bit multiplication
|
| Labels |
new issue
|
| Assignees |
|
| Reporter |
tom-rein
|
LLVM doesn't use mulw, except when it otherwise would have to introduce a sext.w.
The following code could use only mulw, but LLVM uses mul instead ([godbolt](https://godbolt.org/z/9aoTKK8Kv)):
```C++
int cube(int x) {
return x * x * x;
}
int squarep1(int x) {
return x * x + 1;
}
int mulshift(int x) {
return x * x << 4;
}
void store(int x, int &y) {
y = x * x;
}
void store(short x, short &y) {
y = x * x;
}
void store(unsigned char x, unsigned char &y) {
y = x * x;
}
void store(signed char x, signed char &y) {
y = x * x;
}
short square16(short x) {
return x * x;
}
int square(short x) {
return x * x;
}
unsigned short square16(unsigned short x) {
return x * x;
}
unsigned char square8(unsigned char x) {
return x * x;
}
signed char square8(signed char x) {
return x * x;
}
int square(unsigned char x) {
return x * x;
}
int square(signed char x) {
return x * x;
}
long square(unsigned x) {
return x * x;
}
```
LLVM generates this:
```
cube(int):
mul a1, a0, a0
mulw a0, a1, a0
ret
squarep1(int):
mul a0, a0, a0
addiw a0, a0, 1
ret
mulshift(int):
mul a0, a0, a0
slliw a0, a0, 4
ret
store(int, int&):
mul a0, a0, a0
sw a0, 0(a1)
ret
store(short, short&):
mul a0, a0, a0
sh a0, 0(a1)
ret
store(unsigned char, unsigned char&):
mul a0, a0, a0
sb a0, 0(a1)
ret
store(signed char, signed char&):
mul a0, a0, a0
sb a0, 0(a1)
ret
square16(short):
mul a0, a0, a0
sext.h a0, a0
ret
square(short):
mul a0, a0, a0
ret
square16(unsigned short):
mul a0, a0, a0
zext.h a0, a0
ret
square8(unsigned char):
mul a0, a0, a0
zext.b a0, a0
ret
square8(signed char):
mul a0, a0, a0
sext.b a0, a0
ret
square(unsigned char):
mul a0, a0, a0
ret
square(signed char):
mul a0, a0, a0
ret
square(unsigned int):
mul a0, a0, a0
zext.w a0, a0
ret
```
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