[AntonErtl]

The "optimization" was added and turned on by default (for some higher optimization levels) in a pre-release of gcc-4.8. After it turned out to miscompile SPEC, they disabled it for the case occuring there (it still seems to be on by default for other cases); it was disabled in released gcc-4.8 and all following gcc versions. When I asked a gcc maintainer about that, he wrote that it turned out that this "optimization" did not really provide a significant performance benefit; I asked how they evaluated that and he wrote that they used a set of relevant benchmarks. I have seen little reluctance by the gcc maintainers in enabling "optimizations" by default, even if they miscompile real-world programs. My conclusion is that SPEC has a special status among gcc maintainers, but draw your own conclusions.

[DannyBee]

This optimization has been around a long time.

You are wrong about the timeline, it was added in 2012 well before the 4.8 branch point (it's closer to the 4.7 timeline). It was not added in a pre-release of gcc 4.8, and it was actually added to target some common cases. And in fact, it existed before then in other forms (I can point you at code back to 2004 doing the same thing). It has been on by default there for a long time. It happened to not miscompile SPEC before then by luck.

". After it turned out to miscompile SPEC, they disabled it for the case occuring there (it still seems to be on by default for other cases);"

No, it was not. It is on by default for all cases, AFAICT. Please point to the patch you think disabled it. " it was disabled in released gcc-4.8 and all following gcc versions."

Again, i don't see a patch that does this. If this has happened, i imagine it's not on purpose.

Here's the current state of this optimization: from https://gcc.gnu.org/svn/gcc/trunk/gcc/common.opt

faggressive-loop-optimizations Common Report Var(flag_aggressive_loop_optimizations) Optimization Init(1) Aggressively optimize loops using language constraints.

The Init(1) means it's on by default. Note it is mentioned in the release notes for 4.8, for all versions: https://gcc.gnu.org/gcc-4.8/changes.html

Nowhere will you see something that says it is not the default, or will you see changes mentioning the default was changed.

" When I asked a gcc maintainer about that, he wrote that it turned out that this "optimization" did not really provide a significant performance benefit; I asked how they evaluated that and he wrote that they used a set of relevant benchmarks."

Which "gcc maintainer" did you ask? I ask because i'm a listed maintainer for the SCEV related optimizations (see https://gcc.gnu.org/svn/gcc/trunk/MAINTAINERS) and you didn't ask me :)

I asked Sebastian as well, and you didn't ask him.

While compiler vendors do target SPEC, GCC generally does not, and in fact, has pretty much repeatedly refused optimizations that are only useful for SPEC.

FYI: putting optimization in quotes repeatedly does not help your case.

[AntonErtl]

I tested compiling SATD() with gcc-4.8.2 and gcc-5.2.0 with -O3, and they do not "optimize" SATD() into an infinite loop. That some version between 4.7 and 4.8 did actually do it was well publicized, and is also documented in bug report 53073; this bug report was resolved as "INVALID", yet somehow a change was introduced before 4.8 was released that results in compiling SPEC as intended. If you want to know which patch achieved that, you can bisect the commits.

The gcc maintainer who justified that change to me is Andrew Haley. He may not be working on SCEV, whatever that is, but, unlike you, he admits to this change that has provably happened.

[DannyBee]

"I tested compiling SATD() with gcc-4.8.2 and gcc-5.2.0 with -O3, and they do not "optimize" SATD() into an infinite loop."

and from this you assume it was specifically changed (much like you assume a lot of bad faith throughout), instead of "just falling by the wayside as the result of some other change", which is infinitely more likely.

I have no desire to know what patch did it, actually, i'm pointing out you are, pretty much everywhere, going off half-cocked with assumptions and accusations.

"The gcc maintainer who justified that change to me is Andrew Haley. He may not be working on SCEV, whatever that is, but, unlike you, he admits to this change that has provably happened."

Andrew pretty much has only ever worked on the java front end, and that's what he maintains. SCEV is the thing that changed here.

I'm not sure why Andrew would have ever said anything about SPEC (since it's not java ;P), but i actually at this point suspect more bad faith on your part, given what you've said so far.