Philip Reames via llvm-dev
2015-Sep-25 20:26 UTC
[llvm-dev] Comparing stack addresses and function args (Was: [llvm] r174131 - Add a comment explaining an unavailable optimization)
On 09/24/2015 06:04 PM, Hans Wennborg wrote:> I tried your patch on a Clang build to see if it would fire. It > reduced the size of a bootstrap with 8500 bytes. Not huge, but it > seems like a nice improvement. And maybe it could be made more > powerful: not just checking if the address is a param or alloca, but > an address based on such values.Yeah, I realized after posting that this should be integrated with the existing noalias handling in the pointer icmp routine. However, let's defer that until we decide if this is actually correct to implement. :)> > On Thu, Sep 24, 2015 at 2:55 PM, Philip Reames > <listmail at philipreames.com> wrote: >> I threw together a patch which implements this (attached.) If we decide >> that this is actually a legal transform, I'm happy to post this for review. >> >> In addition to the version proposed here, I also implemented a case where a >> trivially escaped alloca's address is not equal to any other value. I >> believe both are valid, but we should confirm. >> >> Philip >> >> >> On 09/24/2015 02:34 PM, Aaron Ballman via llvm-dev wrote: >>> On Thu, Sep 24, 2015 at 5:15 PM, Hans Wennborg <hans at chromium.org> wrote: >>>> On Thu, Sep 24, 2015 at 12:06 PM, Aaron Ballman <aaron at aaronballman.com> >>>> wrote: >>>>> On Thu, Sep 24, 2015 at 2:42 PM, Hans Wennborg <hans at chromium.org> >>>>> wrote: >>>>>> I was wondering why LLVM cannot optimize this code (which GCC does >>>>>> optimize): >>>>>> >>>>>> int f(int *p) { int x; return p == &x; } >>>>>> >>>>>> it would seem that this must always return 0. (This occurs as a >>>>>> self-assignment check in the code I was looking at; I was hoping we >>>>>> could fold that check away.) >>>>> This is different than a self-assignment check, is it not? >>>>> >>>>> blah& operator=(const blah &b) { >>>>> if (&b == this) {} >>>>> // ... >>>>> } >>>>> >>>>> (Because it gets the pointer from the parameter and compares against a >>>>> "local" pointer?) >>>>> >>>>> I just want to make sure that you're not suggesting we should optimize >>>>> away self-assignment checks in the general case. >>>> Right, I'm not suggesting that :-) >>>> >>>> The code I looked at went something like this: >>>> >>>> struct S { >>>> S& operator=(const S& other) { >>>> if (&other != this) >>>> val = other.val; >>>> return *this; >>>> } >>>> void foo(); >>>> int val; >>>> }; >>>> void S::foo() { >>>> S tmp; >>>> tmp.val = 42; >>>> *this = tmp; // operator= gets inlined; we should know(?) that &tmp >>>> != this >>>> } >>>> >>>> This is of course a silly example, but with GCC we get: >>>> >>>> movl $42, (%rdi) >>>> ret >>>> >>>> whereas Clang generates: >>>> >>>> movl $42, -8(%rsp) >>>> leaq -8(%rsp), %rax >>>> cmpq %rdi, %rax >>>> je .LBB0_2 >>>> movl $42, (%rdi) >>>> .LBB0_2: >>>> retq >>>> >>>> which made me sad. >>> Ah, yes, this makes perfect sense to me. Thank you for the explanation! >>> >>> ~Aaron >>> >>>>>> I'd be interested to hear what those with a stronger understanding of >>>>>> the standard than myself think about this, and also if there is any >>>>>> example of something that could break because of this optimization. If >>>>>> not, I'd like us to optimize it :-) >>>>>> >>>>>> >>>>>> On Thu, Jan 31, 2013 at 4:49 PM, Dan Gohman <dan433584 at gmail.com> >>>>>> wrote: >>>>>>> Author: djg >>>>>>> Date: Thu Jan 31 18:49:06 2013 >>>>>>> New Revision: 174131 >>>>>>> >>>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=174131&view=rev >>>>>>> Log: >>>>>>> Add a comment explaining an unavailable optimization. >>>>>>> >>>>>>> Modified: >>>>>>> llvm/trunk/lib/Analysis/InstructionSimplify.cpp >>>>>>> >>>>>>> Modified: llvm/trunk/lib/Analysis/InstructionSimplify.cpp >>>>>>> URL: >>>>>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?rev=174131&r1=174130&r2=174131&view=diff >>>>>>> >>>>>>> =============================================================================>>>>>>> --- llvm/trunk/lib/Analysis/InstructionSimplify.cpp (original) >>>>>>> +++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Thu Jan 31 >>>>>>> 18:49:06 2013 >>>>>>> @@ -1688,6 +1688,34 @@ static Value *ExtractEquivalentCondition >>>>>>> return 0; >>>>>>> } >>>>>>> >>>>>>> +// A significant optimization not implemented here is assuming that >>>>>>> alloca >>>>>>> +// addresses are not equal to incoming argument values. They don't >>>>>>> *alias*, >>>>>>> +// as we say, but that doesn't mean they aren't equal, so we take a >>>>>>> +// conservative approach. >>>>>>> +// >>>>>>> +// This is inspired in part by C++11 5.10p1: >>>>>>> +// "Two pointers of the same type compare equal if and only if they >>>>>>> are both >>>>>>> +// null, both point to the same function, or both represent the >>>>>>> same >>>>>>> +// address." >>>>>>> +// >>>>>>> +// This is pretty permissive. >>>>>> Indeed :-/ >>>>>> >>>>>>> +// It's also partly due to C11 6.5.9p6: >>>>>>> +// "Two pointers compare equal if and only if both are null >>>>>>> pointers, both are >>>>>>> +// pointers to the same object (including a pointer to an object >>>>>>> and a >>>>>>> +// subobject at its beginning) or function, both are pointers to >>>>>>> one past the >>>>>>> +// last element of the same array object, or one is a pointer to >>>>>>> one past the >>>>>>> +// end of one array object and the other is a pointer to the start >>>>>>> of a >>>>>>> +// different array object that happens to immediately follow the >>>>>>> ï¬ rst array >>>>>>> +// object in the address space.) >>>>>>> +// >>>>>>> +// C11's version is more restrictive, however there's no reason why >>>>>>> an argument >>>>>>> +// couldn't be a one-past-the-end value for a stack object in the >>>>>>> caller and be >>>>>>> +// equal to the beginning of a stack object in the callee. >>>>>> This is interesting. >>>>>> >>>>>> For the one-past-the-end pointer to point into the callee, the stack >>>>>> would have to be growing upwards. So this won't happen on X86. Can we >>>>>> turn this optimization on for downward-growing-stack targets? >>>>>> >>>>>> Second, if the stack grows upward, and the function argument does >>>>>> point into the callee stack frame, "p" and "&x" could have the same >>>>>> contents. So per the "represent the same address" part above, they >>>>>> should compare equal? But they're noalias? Are we allowed to write >>>>>> through p? It wasn't a pointer to a valid object when we made the >>>>>> call, but it became valid in the callee? This is all terrifying. >>>>>> >>>>>> I suppose one could store the value of &x though, and then use it >>>>>> again later, i.e.: >>>>>> >>>>>> int *global; >>>>>> int f(int *p) { >>>>>> int x; >>>>>> global = &x; >>>>>> return p == &x; >>>>>> } >>>>>> int g() { >>>>>> f(0); >>>>>> return f(global); >>>>>> } >>>>>> >>>>>> Is g() guaranteed to return 1 here? Maybe we could claim it's >>>>>> implementation dependent? GCC does not seem fold p==&x to 0 here. I >>>>>> suppose we could make sure to check whether &x escapes the function? >>>>>> >>>>>> - Hans >>> _______________________________________________ >>> LLVM Developers mailing list >>> llvm-dev at lists.llvm.org >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >>
Hal Finkel via llvm-dev
2015-Sep-25 22:40 UTC
[llvm-dev] Comparing stack addresses and function args (Was: [llvm] r174131 - Add a comment explaining an unavailable optimization)
----- Original Message -----> From: "Philip Reames via llvm-dev" <llvm-dev at lists.llvm.org> > To: "Hans Wennborg" <hans at chromium.org> > Cc: "llvm-dev" <llvm-dev at lists.llvm.org>, "Dan Gohman" <dan.gohman at gmail.com> > Sent: Friday, September 25, 2015 3:26:01 PM > Subject: Re: [llvm-dev] Comparing stack addresses and function args (Was: [llvm] r174131 - Add a comment explaining > an unavailable optimization) > > > On 09/24/2015 06:04 PM, Hans Wennborg wrote: > > I tried your patch on a Clang build to see if it would fire. It > > reduced the size of a bootstrap with 8500 bytes. Not huge, but it > > seems like a nice improvement. And maybe it could be made more > > powerful: not just checking if the address is a param or alloca, > > but > > an address based on such values. > Yeah, I realized after posting that this should be integrated with > the > existing noalias handling in the pointer icmp routine. However, > let's > defer that until we decide if this is actually correct to implement. > :)Agreed (in your patch, you'd need to call stripPointerCasts() and GetUnderlyingObjects anyway). This should go in computePointerICmp in lib/Analysis/InstructionSimplify.cpp near where IsAllocDisjoint is defined. The optimization seems valid to be, in that we generally prohibit 'getting lucky' with address comparisons; if you couldn't know the address, then guessing doesn't count. -Hal> > > > On Thu, Sep 24, 2015 at 2:55 PM, Philip Reames > > <listmail at philipreames.com> wrote: > >> I threw together a patch which implements this (attached.) If we > >> decide > >> that this is actually a legal transform, I'm happy to post this > >> for review. > >> > >> In addition to the version proposed here, I also implemented a > >> case where a > >> trivially escaped alloca's address is not equal to any other > >> value. I > >> believe both are valid, but we should confirm. > >> > >> Philip > >> > >> > >> On 09/24/2015 02:34 PM, Aaron Ballman via llvm-dev wrote: > >>> On Thu, Sep 24, 2015 at 5:15 PM, Hans Wennborg > >>> <hans at chromium.org> wrote: > >>>> On Thu, Sep 24, 2015 at 12:06 PM, Aaron Ballman > >>>> <aaron at aaronballman.com> > >>>> wrote: > >>>>> On Thu, Sep 24, 2015 at 2:42 PM, Hans Wennborg > >>>>> <hans at chromium.org> > >>>>> wrote: > >>>>>> I was wondering why LLVM cannot optimize this code (which GCC > >>>>>> does > >>>>>> optimize): > >>>>>> > >>>>>> int f(int *p) { int x; return p == &x; } > >>>>>> > >>>>>> it would seem that this must always return 0. (This occurs as > >>>>>> a > >>>>>> self-assignment check in the code I was looking at; I was > >>>>>> hoping we > >>>>>> could fold that check away.) > >>>>> This is different than a self-assignment check, is it not? > >>>>> > >>>>> blah& operator=(const blah &b) { > >>>>> if (&b == this) {} > >>>>> // ... > >>>>> } > >>>>> > >>>>> (Because it gets the pointer from the parameter and compares > >>>>> against a > >>>>> "local" pointer?) > >>>>> > >>>>> I just want to make sure that you're not suggesting we should > >>>>> optimize > >>>>> away self-assignment checks in the general case. > >>>> Right, I'm not suggesting that :-) > >>>> > >>>> The code I looked at went something like this: > >>>> > >>>> struct S { > >>>> S& operator=(const S& other) { > >>>> if (&other != this) > >>>> val = other.val; > >>>> return *this; > >>>> } > >>>> void foo(); > >>>> int val; > >>>> }; > >>>> void S::foo() { > >>>> S tmp; > >>>> tmp.val = 42; > >>>> *this = tmp; // operator= gets inlined; we should know(?) > >>>> that &tmp > >>>> != this > >>>> } > >>>> > >>>> This is of course a silly example, but with GCC we get: > >>>> > >>>> movl $42, (%rdi) > >>>> ret > >>>> > >>>> whereas Clang generates: > >>>> > >>>> movl $42, -8(%rsp) > >>>> leaq -8(%rsp), %rax > >>>> cmpq %rdi, %rax > >>>> je .LBB0_2 > >>>> movl $42, (%rdi) > >>>> .LBB0_2: > >>>> retq > >>>> > >>>> which made me sad. > >>> Ah, yes, this makes perfect sense to me. Thank you for the > >>> explanation! > >>> > >>> ~Aaron > >>> > >>>>>> I'd be interested to hear what those with a stronger > >>>>>> understanding of > >>>>>> the standard than myself think about this, and also if there > >>>>>> is any > >>>>>> example of something that could break because of this > >>>>>> optimization. If > >>>>>> not, I'd like us to optimize it :-) > >>>>>> > >>>>>> > >>>>>> On Thu, Jan 31, 2013 at 4:49 PM, Dan Gohman > >>>>>> <dan433584 at gmail.com> > >>>>>> wrote: > >>>>>>> Author: djg > >>>>>>> Date: Thu Jan 31 18:49:06 2013 > >>>>>>> New Revision: 174131 > >>>>>>> > >>>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=174131&view=rev > >>>>>>> Log: > >>>>>>> Add a comment explaining an unavailable optimization. > >>>>>>> > >>>>>>> Modified: > >>>>>>> llvm/trunk/lib/Analysis/InstructionSimplify.cpp > >>>>>>> > >>>>>>> Modified: llvm/trunk/lib/Analysis/InstructionSimplify.cpp > >>>>>>> URL: > >>>>>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?rev=174131&r1=174130&r2=174131&view=diff > >>>>>>> > >>>>>>> =============================================================================> >>>>>>> --- llvm/trunk/lib/Analysis/InstructionSimplify.cpp > >>>>>>> (original) > >>>>>>> +++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Thu Jan > >>>>>>> 31 > >>>>>>> 18:49:06 2013 > >>>>>>> @@ -1688,6 +1688,34 @@ static Value > >>>>>>> *ExtractEquivalentCondition > >>>>>>> return 0; > >>>>>>> } > >>>>>>> > >>>>>>> +// A significant optimization not implemented here is > >>>>>>> assuming that > >>>>>>> alloca > >>>>>>> +// addresses are not equal to incoming argument values. They > >>>>>>> don't > >>>>>>> *alias*, > >>>>>>> +// as we say, but that doesn't mean they aren't equal, so we > >>>>>>> take a > >>>>>>> +// conservative approach. > >>>>>>> +// > >>>>>>> +// This is inspired in part by C++11 5.10p1: > >>>>>>> +// "Two pointers of the same type compare equal if and > >>>>>>> only if they > >>>>>>> are both > >>>>>>> +// null, both point to the same function, or both > >>>>>>> represent the > >>>>>>> same > >>>>>>> +// address." > >>>>>>> +// > >>>>>>> +// This is pretty permissive. > >>>>>> Indeed :-/ > >>>>>> > >>>>>>> +// It's also partly due to C11 6.5.9p6: > >>>>>>> +// "Two pointers compare equal if and only if both are > >>>>>>> null > >>>>>>> pointers, both are > >>>>>>> +// pointers to the same object (including a pointer to an > >>>>>>> object > >>>>>>> and a > >>>>>>> +// subobject at its beginning) or function, both are > >>>>>>> pointers to > >>>>>>> one past the > >>>>>>> +// last element of the same array object, or one is a > >>>>>>> pointer to > >>>>>>> one past the > >>>>>>> +// end of one array object and the other is a pointer to > >>>>>>> the start > >>>>>>> of a > >>>>>>> +// different array object that happens to immediately > >>>>>>> follow the > >>>>>>> ï¬ rst array > >>>>>>> +// object in the address space.) > >>>>>>> +// > >>>>>>> +// C11's version is more restrictive, however there's no > >>>>>>> reason why > >>>>>>> an argument > >>>>>>> +// couldn't be a one-past-the-end value for a stack object > >>>>>>> in the > >>>>>>> caller and be > >>>>>>> +// equal to the beginning of a stack object in the callee. > >>>>>> This is interesting. > >>>>>> > >>>>>> For the one-past-the-end pointer to point into the callee, the > >>>>>> stack > >>>>>> would have to be growing upwards. So this won't happen on X86. > >>>>>> Can we > >>>>>> turn this optimization on for downward-growing-stack targets? > >>>>>> > >>>>>> Second, if the stack grows upward, and the function argument > >>>>>> does > >>>>>> point into the callee stack frame, "p" and "&x" could have the > >>>>>> same > >>>>>> contents. So per the "represent the same address" part above, > >>>>>> they > >>>>>> should compare equal? But they're noalias? Are we allowed to > >>>>>> write > >>>>>> through p? It wasn't a pointer to a valid object when we made > >>>>>> the > >>>>>> call, but it became valid in the callee? This is all > >>>>>> terrifying. > >>>>>> > >>>>>> I suppose one could store the value of &x though, and then use > >>>>>> it > >>>>>> again later, i.e.: > >>>>>> > >>>>>> int *global; > >>>>>> int f(int *p) { > >>>>>> int x; > >>>>>> global = &x; > >>>>>> return p == &x; > >>>>>> } > >>>>>> int g() { > >>>>>> f(0); > >>>>>> return f(global); > >>>>>> } > >>>>>> > >>>>>> Is g() guaranteed to return 1 here? Maybe we could claim it's > >>>>>> implementation dependent? GCC does not seem fold p==&x to 0 > >>>>>> here. I > >>>>>> suppose we could make sure to check whether &x escapes the > >>>>>> function? > >>>>>> > >>>>>> - Hans > >>> _______________________________________________ > >>> LLVM Developers mailing list > >>> llvm-dev at lists.llvm.org > >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev > >> > > _______________________________________________ > LLVM Developers mailing list > llvm-dev at lists.llvm.org > http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >-- Hal Finkel Assistant Computational Scientist Leadership Computing Facility Argonne National Laboratory
Hans Wennborg via llvm-dev
2015-Sep-28 18:09 UTC
[llvm-dev] Comparing stack addresses and function args (Was: [llvm] r174131 - Add a comment explaining an unavailable optimization)
On Fri, Sep 25, 2015 at 3:40 PM, Hal Finkel <hfinkel at anl.gov> wrote:> ----- Original Message ----- >> From: "Philip Reames via llvm-dev" <llvm-dev at lists.llvm.org> >> To: "Hans Wennborg" <hans at chromium.org> >> Cc: "llvm-dev" <llvm-dev at lists.llvm.org>, "Dan Gohman" <dan.gohman at gmail.com> >> Sent: Friday, September 25, 2015 3:26:01 PM >> Subject: Re: [llvm-dev] Comparing stack addresses and function args (Was: [llvm] r174131 - Add a comment explaining >> an unavailable optimization) >> >> >> On 09/24/2015 06:04 PM, Hans Wennborg wrote: >> > I tried your patch on a Clang build to see if it would fire. It >> > reduced the size of a bootstrap with 8500 bytes. Not huge, but it >> > seems like a nice improvement. And maybe it could be made more >> > powerful: not just checking if the address is a param or alloca, >> > but >> > an address based on such values. >> Yeah, I realized after posting that this should be integrated with >> the >> existing noalias handling in the pointer icmp routine. However, >> let's >> defer that until we decide if this is actually correct to implement. >> :) > > Agreed (in your patch, you'd need to call stripPointerCasts() and GetUnderlyingObjects anyway). This should go in computePointerICmp in lib/Analysis/InstructionSimplify.cpp near where IsAllocDisjoint is defined. The optimization seems valid to be, in that we generally prohibit 'getting lucky' with address comparisons; if you couldn't know the address, then guessing doesn't count.I do think we'll need to be a little bit careful here, though. I think we can optimize this by acting as-if no one could know the address of a stack variable, but then we need to be able to do so consistently. For example, in this code int f(int *p) { int x; bool a = (p == &x); bool b = equals(p, &x); } a and b needs to end up with the same value.>> > On Thu, Sep 24, 2015 at 2:55 PM, Philip Reames >> > <listmail at philipreames.com> wrote: >> >> I threw together a patch which implements this (attached.) If we >> >> decide >> >> that this is actually a legal transform, I'm happy to post this >> >> for review. >> >> >> >> In addition to the version proposed here, I also implemented a >> >> case where a >> >> trivially escaped alloca's address is not equal to any other >> >> value. I >> >> believe both are valid, but we should confirm. >> >> >> >> Philip >> >> >> >> >> >> On 09/24/2015 02:34 PM, Aaron Ballman via llvm-dev wrote: >> >>> On Thu, Sep 24, 2015 at 5:15 PM, Hans Wennborg >> >>> <hans at chromium.org> wrote: >> >>>> On Thu, Sep 24, 2015 at 12:06 PM, Aaron Ballman >> >>>> <aaron at aaronballman.com> >> >>>> wrote: >> >>>>> On Thu, Sep 24, 2015 at 2:42 PM, Hans Wennborg >> >>>>> <hans at chromium.org> >> >>>>> wrote: >> >>>>>> I was wondering why LLVM cannot optimize this code (which GCC >> >>>>>> does >> >>>>>> optimize): >> >>>>>> >> >>>>>> int f(int *p) { int x; return p == &x; } >> >>>>>> >> >>>>>> it would seem that this must always return 0. (This occurs as >> >>>>>> a >> >>>>>> self-assignment check in the code I was looking at; I was >> >>>>>> hoping we >> >>>>>> could fold that check away.) >> >>>>> This is different than a self-assignment check, is it not? >> >>>>> >> >>>>> blah& operator=(const blah &b) { >> >>>>> if (&b == this) {} >> >>>>> // ... >> >>>>> } >> >>>>> >> >>>>> (Because it gets the pointer from the parameter and compares >> >>>>> against a >> >>>>> "local" pointer?) >> >>>>> >> >>>>> I just want to make sure that you're not suggesting we should >> >>>>> optimize >> >>>>> away self-assignment checks in the general case. >> >>>> Right, I'm not suggesting that :-) >> >>>> >> >>>> The code I looked at went something like this: >> >>>> >> >>>> struct S { >> >>>> S& operator=(const S& other) { >> >>>> if (&other != this) >> >>>> val = other.val; >> >>>> return *this; >> >>>> } >> >>>> void foo(); >> >>>> int val; >> >>>> }; >> >>>> void S::foo() { >> >>>> S tmp; >> >>>> tmp.val = 42; >> >>>> *this = tmp; // operator= gets inlined; we should know(?) >> >>>> that &tmp >> >>>> != this >> >>>> } >> >>>> >> >>>> This is of course a silly example, but with GCC we get: >> >>>> >> >>>> movl $42, (%rdi) >> >>>> ret >> >>>> >> >>>> whereas Clang generates: >> >>>> >> >>>> movl $42, -8(%rsp) >> >>>> leaq -8(%rsp), %rax >> >>>> cmpq %rdi, %rax >> >>>> je .LBB0_2 >> >>>> movl $42, (%rdi) >> >>>> .LBB0_2: >> >>>> retq >> >>>> >> >>>> which made me sad. >> >>> Ah, yes, this makes perfect sense to me. Thank you for the >> >>> explanation! >> >>> >> >>> ~Aaron >> >>> >> >>>>>> I'd be interested to hear what those with a stronger >> >>>>>> understanding of >> >>>>>> the standard than myself think about this, and also if there >> >>>>>> is any >> >>>>>> example of something that could break because of this >> >>>>>> optimization. If >> >>>>>> not, I'd like us to optimize it :-) >> >>>>>> >> >>>>>> >> >>>>>> On Thu, Jan 31, 2013 at 4:49 PM, Dan Gohman >> >>>>>> <dan433584 at gmail.com> >> >>>>>> wrote: >> >>>>>>> Author: djg >> >>>>>>> Date: Thu Jan 31 18:49:06 2013 >> >>>>>>> New Revision: 174131 >> >>>>>>> >> >>>>>>> URL: http://llvm.org/viewvc/llvm-project?rev=174131&view=rev >> >>>>>>> Log: >> >>>>>>> Add a comment explaining an unavailable optimization. >> >>>>>>> >> >>>>>>> Modified: >> >>>>>>> llvm/trunk/lib/Analysis/InstructionSimplify.cpp >> >>>>>>> >> >>>>>>> Modified: llvm/trunk/lib/Analysis/InstructionSimplify.cpp >> >>>>>>> URL: >> >>>>>>> http://llvm.org/viewvc/llvm-project/llvm/trunk/lib/Analysis/InstructionSimplify.cpp?rev=174131&r1=174130&r2=174131&view=diff >> >>>>>>> >> >>>>>>> =============================================================================>> >>>>>>> --- llvm/trunk/lib/Analysis/InstructionSimplify.cpp >> >>>>>>> (original) >> >>>>>>> +++ llvm/trunk/lib/Analysis/InstructionSimplify.cpp Thu Jan >> >>>>>>> 31 >> >>>>>>> 18:49:06 2013 >> >>>>>>> @@ -1688,6 +1688,34 @@ static Value >> >>>>>>> *ExtractEquivalentCondition >> >>>>>>> return 0; >> >>>>>>> } >> >>>>>>> >> >>>>>>> +// A significant optimization not implemented here is >> >>>>>>> assuming that >> >>>>>>> alloca >> >>>>>>> +// addresses are not equal to incoming argument values. They >> >>>>>>> don't >> >>>>>>> *alias*, >> >>>>>>> +// as we say, but that doesn't mean they aren't equal, so we >> >>>>>>> take a >> >>>>>>> +// conservative approach. >> >>>>>>> +// >> >>>>>>> +// This is inspired in part by C++11 5.10p1: >> >>>>>>> +// "Two pointers of the same type compare equal if and >> >>>>>>> only if they >> >>>>>>> are both >> >>>>>>> +// null, both point to the same function, or both >> >>>>>>> represent the >> >>>>>>> same >> >>>>>>> +// address." >> >>>>>>> +// >> >>>>>>> +// This is pretty permissive. >> >>>>>> Indeed :-/ >> >>>>>> >> >>>>>>> +// It's also partly due to C11 6.5.9p6: >> >>>>>>> +// "Two pointers compare equal if and only if both are >> >>>>>>> null >> >>>>>>> pointers, both are >> >>>>>>> +// pointers to the same object (including a pointer to an >> >>>>>>> object >> >>>>>>> and a >> >>>>>>> +// subobject at its beginning) or function, both are >> >>>>>>> pointers to >> >>>>>>> one past the >> >>>>>>> +// last element of the same array object, or one is a >> >>>>>>> pointer to >> >>>>>>> one past the >> >>>>>>> +// end of one array object and the other is a pointer to >> >>>>>>> the start >> >>>>>>> of a >> >>>>>>> +// different array object that happens to immediately >> >>>>>>> follow the >> >>>>>>> ï¬ rst array >> >>>>>>> +// object in the address space.) >> >>>>>>> +// >> >>>>>>> +// C11's version is more restrictive, however there's no >> >>>>>>> reason why >> >>>>>>> an argument >> >>>>>>> +// couldn't be a one-past-the-end value for a stack object >> >>>>>>> in the >> >>>>>>> caller and be >> >>>>>>> +// equal to the beginning of a stack object in the callee. >> >>>>>> This is interesting. >> >>>>>> >> >>>>>> For the one-past-the-end pointer to point into the callee, the >> >>>>>> stack >> >>>>>> would have to be growing upwards. So this won't happen on X86. >> >>>>>> Can we >> >>>>>> turn this optimization on for downward-growing-stack targets? >> >>>>>> >> >>>>>> Second, if the stack grows upward, and the function argument >> >>>>>> does >> >>>>>> point into the callee stack frame, "p" and "&x" could have the >> >>>>>> same >> >>>>>> contents. So per the "represent the same address" part above, >> >>>>>> they >> >>>>>> should compare equal? But they're noalias? Are we allowed to >> >>>>>> write >> >>>>>> through p? It wasn't a pointer to a valid object when we made >> >>>>>> the >> >>>>>> call, but it became valid in the callee? This is all >> >>>>>> terrifying. >> >>>>>> >> >>>>>> I suppose one could store the value of &x though, and then use >> >>>>>> it >> >>>>>> again later, i.e.: >> >>>>>> >> >>>>>> int *global; >> >>>>>> int f(int *p) { >> >>>>>> int x; >> >>>>>> global = &x; >> >>>>>> return p == &x; >> >>>>>> } >> >>>>>> int g() { >> >>>>>> f(0); >> >>>>>> return f(global); >> >>>>>> } >> >>>>>> >> >>>>>> Is g() guaranteed to return 1 here? Maybe we could claim it's >> >>>>>> implementation dependent? GCC does not seem fold p==&x to 0 >> >>>>>> here. I >> >>>>>> suppose we could make sure to check whether &x escapes the >> >>>>>> function? >> >>>>>> >> >>>>>> - Hans >> >>> _______________________________________________ >> >>> LLVM Developers mailing list >> >>> llvm-dev at lists.llvm.org >> >>> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >> >> >> >> _______________________________________________ >> LLVM Developers mailing list >> llvm-dev at lists.llvm.org >> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev >> > > -- > Hal Finkel > Assistant Computational Scientist > Leadership Computing Facility > Argonne National Laboratory