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digitalmars.D - Optimizing out unnecessary allocations

reply "David Nadlinger" <see klickverbot.at> writes:
I recently revived an LDC optimization pass (originally written 
by Frits van Bommel) that can remove dead allocations and promote 
small GC allocations to stack memory by recognizing the related 
druntime calls.

However, I didn't commit it to the main branch yet because it 
causes a test case in the DMD test suite to fail: 
https://github.com/D-Programming-Language/dmd/blob/eaa03fefeb1a698f586d5f5a09068f3433bf4b29/test/runnable/testgc2.d

The test is intended to make sure that the garbage collector 
properly handles out-of-memory situations, but in doing so 
assumes that the compiler can't touch the array allocations. 
However, when building with optimizations on, LDC recognizes the 
fact that the allocations are unused and subsequently removes 
them, causing the "assert(0)"s to be hit:

———
try {
   long[] l = new long[ptrdiff_t.max];
   assert(0);
}
catch (OutOfMemoryError o){}
———

Thus, my question: Is it legal for a D compiler to eliminate dead 
GC allocations (given that there are no side effects due to 
constructors, …)?

I'd strongly argue in favor of such optimizations to be legal, 
because not allowing them would eliminate quite a few 
optimization possibilities. There is also precedent for such 
behavior; many C compilers treat malloc() in a similar fashion.

David
Oct 17 2012
next sibling parent reply Artur Skawina <art.08.09 gmail.com> writes:
On 10/17/12 20:10, David Nadlinger wrote:
 I recently revived an LDC optimization pass (originally written by Frits van
Bommel) that can remove dead allocations and promote small GC allocations to
stack memory by recognizing the related druntime calls.
 
 However, I didn't commit it to the main branch yet because it causes a test
case in the DMD test suite to fail:
https://github.com/D-Programming-Language/dmd/blob/eaa03fefeb1a698f586d5f5a09068f3433bf4b29/test/runnable/testgc2.d
 
 The test is intended to make sure that the garbage collector properly handles
out-of-memory situations, but in doing so assumes that the compiler can't touch
the array allocations. However, when building with optimizations on, LDC
recognizes the fact that the allocations are unused and subsequently removes
them, causing the "assert(0)"s to be hit:
 
 ———
 try {
   long[] l = new long[ptrdiff_t.max];
   assert(0);
 }
 catch (OutOfMemoryError o){}
 ———
 
 Thus, my question: Is it legal for a D compiler to eliminate dead GC
allocations (given that there are no side effects due to constructors, …)?
 
 I'd strongly argue in favor of such optimizations to be legal, because not
allowing them would eliminate quite a few optimization possibilities. There is
also precedent for such behavior; many C compilers treat malloc() in a similar
fashion.

Well, I think such optimizations are fine (as long as documented and there exists alternatives), but note that this testcase checks for the case where the object size calculation overflows. Ie it must not succeed. Would ignoring the error when nothing accesses the object make sense? artur
Oct 17 2012
parent Artur Skawina <art.08.09 gmail.com> writes:
On 10/17/12 23:00, David Nadlinger wrote:
 On Wednesday, 17 October 2012 at 20:37:53 UTC, Artur Skawina wrote:
 Well, I think such optimizations are fine (as long as documented and there
 exists alternatives), but note that this testcase checks for the case where
 the object size calculation overflows. Ie it must not succeed.

Could you elaborate on that? It strikes me that this is either a GC implementation detail or invalid D code in the first place (i.e. should not be expected to compile resp. is undefined behavior).

Well, eg on a 32-bit platform the newly allocated memory object would need to have a size of 8*2G == 16G. I guess you could see it as a GC implementation detail, but that allocation can never succeed, simply because such an object would be larger than the available address space, hence can't be mapped directly. The 'new long[ptrdiff_t.max]' case can be caught at compile time, but a different 'new long[runtime_variable_which_happens_to_be_2G]' can not, and then the GC MUST catch the overflow, instead of allocating a ((size_t)long.sizeof*2G) sized object Which is what I assume that test was meant to check. But even in the constant, statically-checkable case, would it make sense to ignore the error if the allocation was "dead"? If nothing ever accesses the new object, ignoring the error seems harmless. But is it OK to allow faulty code to silently run long as the compiler can prove that the bug won't be triggered? Will every compiler make the same decision? Would a different optimization level cause the error to be thrown? For these reasons, silently optimizing away "harmless", but buggy code is not a good idea. artur
Oct 17 2012
prev sibling next sibling parent "David Nadlinger" <see klickverbot.at> writes:
On Wednesday, 17 October 2012 at 20:37:53 UTC, Artur Skawina 
wrote:
 Well, I think such optimizations are fine (as long as 
 documented and there
 exists alternatives), but note that this testcase checks for 
 the case where
 the object size calculation overflows. Ie it must not succeed.

Could you elaborate on that? It strikes me that this is either a GC implementation detail or invalid D code in the first place (i.e. should not be expected to compile resp. is undefined behavior). David
Oct 17 2012
prev sibling next sibling parent "David Nadlinger" <see klickverbot.at> writes:
On Wednesday, 17 October 2012 at 18:10:26 UTC, David Nadlinger 
wrote:
 Thus, my question: Is it legal for a D compiler to eliminate 
 dead GC allocations (given that there are no side effects due 
 to constructors, …)?

Walter confirmed that this should be allowed. Pull request fixing the test: https://github.com/D-Programming-Language/dmd/pull/1191 David
Oct 17 2012
prev sibling parent Walter Bright <newshound2 digitalmars.com> writes:
On 10/17/2012 11:10 AM, David Nadlinger wrote:
 Thus, my question: Is it legal for a D compiler to eliminate dead GC
allocations
 (given that there are no side effects due to constructors, …)?

Yes. The test case can be fixed by attempting to use the result of new.
Oct 17 2012