| commit | dae5c4e1e7deef1fddad6e83d0f109b6fc9c5cc4 | [log] [tgz] |
|---|---|---|
| author | Iris Shi <0.0@owo.li> | Fri May 16 09:40:11 2025 +0800 |
| committer | GitHub <noreply@github.com> | Fri May 16 09:40:11 2025 +0800 |
| tree | e746626a491f7aeb819dc22f26aeecd34f3b9736 | |
| parent | 8d63afbe5c8c5856ab4c2e663c1076313001565d [diff] |
[RISCV] Expand constant multiplication for targets without M extension (#137195)
Closes #137023
On RISC-V machines without a native multiply instruction (e.g., `rv32i`
base), multiplying a variable by a constant integer often compiles to a
call to a library routine like `__mul{s,d}i3`.
```assembly
.globl __mulxi3
.type __mulxi3, @function
__mulxi3:
mv a2, a0
mv a0, zero
.L1:
andi a3, a1, 1
beqz a3, .L2
add a0, a0, a2
.L2:
srli a1, a1, 1
slli a2, a2, 1
bnez a1, .L1
ret
```
This library function implements multiplication in software using a loop
of shifts and adds, processing the constant bit by bit. On rv32i, it
requires a minimum of 8 instructions (for multiply by `0`) and up to
about 200 instructions (by `0xffffffff`), involves heavy branching and
function call overhead.
When not optimizing for size, we could expand the constant
multiplication into a sequence of shift and add/sub instructions. For
now we use non-adjacent form for the shift and add/sub sequence, which
could save 1/2 - 2/3 instructions compared to a shl+add-only sequence.
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