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Nelson Elhage nelhage

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nelhage / .gitignore
Last active May 26, 2020
cuviper/probe bug report
View gist:be3c7291c8af79e5fcdf6d40ebb7df98
[nelhage@monolithique:~/code/alexbench]$ clang-11 -O1 -S -c -emit-llvm overflow_2.c
[nelhage@monolithique:~/code/alexbench]$ opt-11 --opt-bisect-limit=137 -O2 -S overflow_2.ll 2>/dev/null | opt-11 -O2 -S -o overflow_2_opt.ll
[nelhage@monolithique:~/code/alexbench]$ cat overflow_2_opt.ll
; ModuleID = '<stdin>'
source_filename = "overflow_2.c"
target datalayout = "e-m:e-p270:32:32-p271:32:32-p272:64:64-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-linux-gnu"
; Function Attrs: nofree nounwind uwtable
define dso_local { i8*, i64 } @f1_overflow() local_unnamed_addr #0 {
nelhage / Cargo.toml
Last active May 4, 2020 — forked from alex/Cargo.toml
vectorized contains4 implementation in rust
View Cargo.toml
name = "f"
version = "0.1.0"
authors = ["Alex Gaynor <>"]
edition = "2018"
# See more keys and their definitions at
packed_simd = "0.3"
View gist:3552913fc013bb7e81b16a9e852d793f
│ intrinsics::move_val_init(&mut *dst, src) ▒
0.90 │ lea (%rsi,%rsi,4),%rcx ▒
0.05 │ shl $0x4,%rcx ▒
0.11 │ mov 0xb8(%rsp),%rdx ▒
0.80 │ mov %rdx,(%rax,%rcx,1) ▒
1.66 │ movaps 0xd0(%rsp),%xmm0 ▒
11.08 │ movups %xmm0,0x8(%rax,%rcx,1)

My approach:

  • Let's use "cost" as a proxy for CO2 emissions, because within an order of magnitude I expect $1 on gas emits as much CO2 as $1 of power for a server, and I believe the bulk of the ongoing cost of operating a server is power budget.
  • Lifetime cost of a gas for a car: Let's estimate 100,000 miles, and 20 mpg and $4/gal. That gives us 5,000 gal and $20,000 in lifetime gas cost for a car. These numbers all feel pessimistic so we're maybe high.
  • A beefy server in AWS or GCP costs something like $1/hr, probably more for the really big ones. So in order to get to $20,000/run, we need 20,000 instance-hours. Let's ballpark a large training run at 1 day, so that's ~800 instances for one day. That feels within the realm of the numbers I've seen in papers.

Conclusion: Entirely plausible. Other conclusion: Lifetime cost of gasoline for a car is (in round numbers) the same as the cost of a new car. I hadn't realized they were so close. Other other conclusion: Now I understand why OpenAI needs billions o

nelhage / main.go.c
Last active Sep 22, 2020
A Go/C polyglot
View main.go.c
// \
#include <stdio.h>
int main() {
printf("Hello, World from C!\n");
return 0;
#if 0
View gist:44da26d0b93a038cdc0a577783a52e1e
#include <stdio.h>
#include <math.h>
#include <stdint.h>
static float rt(float x) {
volatile double g = static_cast<double>(x);
return static_cast<float>(g);
int main() {
#include <thread>
#include <experimental/optional>
#include <atomic>
#include <shared_mutex>
#include <mutex>
#include <iostream>
#include <vector>
using namespace std;
nelhage /
Created Apr 26, 2018
Experience report: Trying to map an `FnMut` over a binary tree in Rust

This story is simplified from an attempt to write an AST walker for a toy compiler, but the essential facts are unchanged.

I am fairly new to Rust (but an experienced C++ programmer) and am trying to write a program to map an FnMut over a binary tree. I start with a simple tree definition, and try to write what seems to me to be the straightforward code:

use std::rc::Rc;
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