4 ms·

But how much actual performance is gained here?

bagels 2y ago
It all adds up. All those instructions you don't have to execute, especially memory access and cache misses from jumps, pipeline stalls from conditionals, not just from this optimization.
ncruces 2y ago
Imagine that you created a function GetPixel that reads an RGB pixel at a memory address, and which has a NULL check as a precondition. If the compiler can "prove" that the pointer is not NULL it can (after inlining the call) remove 20 million checks for a 20 megapixel image. The silly issue is the compiler using "you accessed it before" (aka "undefined behaviour") to "prove" that the pointer is not NULL. But I can attest that avoiding 20 million such checks does indeed make a huge difference.
- Dylan16807 2y ago
  If we're inlining the call, then we can hoist the NULL check out of the loop. Now it's 1 check per 20 million operations. There's no need to eliminate it or have UB at that point.
- cv5005 2y ago
  Just make a non null checking version: GetPixelUnsafe() and let the responsibility onto the user to do the null check before the loop. All of these 'problems' have simple and straigtforward workarounds, I'm not convinced these UB are needed at all.
  - ncruces 2y ago
    That's a non solution for existing code that already calls GetPixel 20 million times. It's not like I'm saying C is the best possible way to write new code. I'm just commenting why this matters for performance, and “remove all undefined behavior" from C compilers is a non-starter. Now go write Rust for all I care.
  - nemothekid 2y ago
    >All of these 'problems' have simple and straigtforward workarounds, I'm not convinced these UB are needed at all. He gave you a simple and straightforward example, but that example may not be representative of a real world program where complex analysis leads to better performing code. As a programmer, its far easier to just insert bounds checks everywhere, and trust the system to remove them when possible. This is what Rust does, and it safe. The problem isn't the compiler, the problem is the standard. More broadly, the standard wasn't written with optimizing compilers in mind.
menaerus 2y ago
It depends on your CPU microarchitectural details, on the complexity and size of your binary executable and the workload of your binary. So there's no universal answer to your question but it could very well be "much".

It all adds up. All those instructions you don't have to execute, especially memory access and cache misses from jumps, pipeline stalls from conditionals, not just from this optimization.

ncruces 2y ago

Imagine that you created a function GetPixel that reads an RGB pixel at a memory address, and which has a NULL check as a precondition. If the compiler can "prove" that the pointer is not NULL it can (after inlining the call) remove 20 million checks for a 20 megapixel image. The silly issue is the compiler using "you accessed it before" (aka "undefined behaviour") to "prove" that the pointer is not NULL. But I can attest that avoiding 20 million such checks does indeed make a huge difference.

Dylan16807 2y ago
If we're inlining the call, then we can hoist the NULL check out of the loop. Now it's 1 check per 20 million operations. There's no need to eliminate it or have UB at that point.
cv5005 2y ago
Just make a non null checking version: GetPixelUnsafe() and let the responsibility onto the user to do the null check before the loop. All of these 'problems' have simple and straigtforward workarounds, I'm not convinced these UB are needed at all.
- ncruces 2y ago
  That's a non solution for existing code that already calls GetPixel 20 million times. It's not like I'm saying C is the best possible way to write new code. I'm just commenting why this matters for performance, and “remove all undefined behavior" from C compilers is a non-starter. Now go write Rust for all I care.
- nemothekid 2y ago
  >All of these 'problems' have simple and straigtforward workarounds, I'm not convinced these UB are needed at all. He gave you a simple and straightforward example, but that example may not be representative of a real world program where complex analysis leads to better performing code. As a programmer, its far easier to just insert bounds checks everywhere, and trust the system to remove them when possible. This is what Rust does, and it safe. The problem isn't the compiler, the problem is the standard. More broadly, the standard wasn't written with optimizing compilers in mind.

menaerus 2y ago

It depends on your CPU microarchitectural details, on the complexity and size of your binary executable and the workload of your binary. So there's no universal answer to your question but it could very well be "much".