Alexey Zhikhartsev via llvm-dev
2020-Feb-14 21:52 UTC
[llvm-dev] Given one restrict pointer based on another, should they never alias?
We recently found an issue when using the full restrict implementation developed by Jeroen; it surfaces when compiling an obscure combination of std::valarray and std::indirect_array but I don't want to bore you with all the details. What it boils down to is this basic question about restrict: Given one restrict pointer based on another, should they never alias? As far as I understand the formal definition of "restrict" in section 6.7.3.1 of the C standard [1], in the function below, pointer `y` is based on "restrict" pointer `x`; hence, the compiler will assume that accesses *x and *y might alias: void assign1(int *pA, long N) { int *restrict x = pA; { int *y = x + N; *x = *y; } } However, what if y itself is declared "restrict": can the compiler assume that *x and *y will never alias? void assign2(int *pA, long N) { int *restrict x = pA; { int *restrict y = x + N; *x = *y; } } Both Jeroen's and Hal's implementation (the intrinsic-based one) will say "NoAlias" for the accesses in assign2() but shouldn't x and y be in the same restrictness "bucket" since y is based on x? [1] http://port70.net/~nsz/c/c11/n1570.html#6.7.3.1 -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200214/f91a6da6/attachment.html>
Jeroen Dobbelaere via llvm-dev
2020-Feb-15 14:00 UTC
[llvm-dev] Given one restrict pointer based on another, should they never alias?
Hi Alexey, This is defined in 6.7.3.1 paragraph 4: '... Every other lvalue used to access the value of X shall also have its address based on P ...' For 'assign1': - x is a restrict pointer and is assumed to point to its own set of objects - y is a normal pointer, based on x - all access to the set of objects pointed by x are done through a pointer based on x for 'assign2': - x is a restrict pointer and is assumed to point to its own set of objects - y is also a restrict pointer, based on x, but it is assumed to point to its own set of objects for the scope of y - because of that, *x and *y must never overlap, as all accesses to the objects of y must be done based on a pointer derived from y As such, a N=0 will trigger undefined behavior in assign2 Doing the assignment to *x outside the inner block, makes the code valid again: void assign3(int *pA, long N) { int *restrict x = pA; int tmp; { int *restrict y = x + N; tmp = *y; } *x = tmp; // may alias with *y } Greetings, Jeroen Dobbelaere From: Alexey Zhikhartsev <alexey.zhikhar at gmail.com> Sent: Friday, February 14, 2020 22:52 To: via Llvm-dev <llvm-dev at lists.llvm.org> Cc: Jeroen Dobbelaere <dobbel at synopsys.com>; hfinkel at anl.gov Subject: Given one restrict pointer based on another, should they never alias? We recently found an issue when using the full restrict implementation developed by Jeroen; it surfaces when compiling an obscure combination of std::valarray and std::indirect_array but I don't want to bore you with all the details. What it boils down to is this basic question about restrict: Given one restrict pointer based on another, should they never alias? As far as I understand the formal definition of "restrict" in section 6.7.3.1 of the C standard [1], in the function below, pointer `y` is based on "restrict" pointer `x`; hence, the compiler will assume that accesses *x and *y might alias: void assign1(int *pA, long N) { int *restrict x = pA; { int *y = x + N; *x = *y; } } However, what if y itself is declared "restrict": can the compiler assume that *x and *y will never alias? void assign2(int *pA, long N) { int *restrict x = pA; { int *restrict y = x + N; *x = *y; } } Both Jeroen's and Hal's implementation (the intrinsic-based one) will say "NoAlias" for the accesses in assign2() but shouldn't x and y be in the same restrictness "bucket" since y is based on x? [1] http://port70.net/~nsz/c/c11/n1570.html#6.7.3.1<https://urldefense.proofpoint.com/v2/url?u=http-3A__port70.net_-7Ensz_c_c11_n1570.html-236.7.3.1&d=DwMFaQ&c=DPL6_X_6JkXFx7AXWqB0tg&r=ELyOnT0WepII6UnFk-OSzxlGOXXSfAvOLT6E8iPwwJk&m=xMDqkSAlj-YCOS4JMDXAENpBS-eaCcLYSkIm1qK68fs&s=B3LRzqpd9bD1724nvhG0FtpFh3QPsQ4FTBGQ4qJn1cA&e=> -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200215/123add23/attachment.html>
Alexey Zhikhartsev via llvm-dev
2020-Feb-20 00:55 UTC
[llvm-dev] Given one restrict pointer based on another, should they never alias?
Thanks, Jeroen, that really helps. A follow-up question, if you don't mind. What if we have code somewhat similar to your example in assign3() but it's in C++ and the pointer derived from x is stored in a class member field: class S { public: S(int *d): data(d) {} int *getData() { return data; } private: int *__restrict__ data; }; void assign4(int *pA, long N) { int *__restrict__ x = pA; int tmp; { S s(x + N); tmp = *s.getData(); } *x = tmp; } I see that the full restrict implementation says that the load and the store do not alias. Is this by design? On Sat, Feb 15, 2020 at 9:00 AM Jeroen Dobbelaere < Jeroen.Dobbelaere at synopsys.com> wrote:> Hi Alexey, > > > > This is defined in 6.7.3.1 paragraph 4: > > '... Every other lvalue used to access the value of X shall also have its > address based on P ...' > > > > For 'assign1': > > - x is a restrict pointer and is assumed to point to its own set of objects > > - y is a normal pointer, based on x > > - all access to the set of objects pointed by x are done through a pointer > based on x > > > > for 'assign2': > > - x is a restrict pointer and is assumed to point to its own set of objects > > - y is also a restrict pointer, based on x, but it is assumed to point to > its own set of objects for the scope of y > > - because of that, *x and *y must never overlap, as all accesses to the > objects of y must be done based on a pointer derived from y > > > > As such, a N=0 will trigger undefined behavior in assign2 > > > > Doing the assignment to *x outside the inner block, makes the code valid > again: > > > > void assign3(int *pA, long N) { > int *restrict x = pA; > > int tmp; > { > int *restrict y = x + N; > tmp = *y; > } > > *x = tmp; // may alias with *y > } > > Greetings, > > > > Jeroen Dobbelaere > > > > > > > > *From:* Alexey Zhikhartsev <alexey.zhikhar at gmail.com> > *Sent:* Friday, February 14, 2020 22:52 > *To:* via Llvm-dev <llvm-dev at lists.llvm.org> > *Cc:* Jeroen Dobbelaere <dobbel at synopsys.com>; hfinkel at anl.gov > *Subject:* Given one restrict pointer based on another, should they never > alias? > > > > We recently found an issue when using the full restrict implementation > developed by Jeroen; it surfaces when compiling an obscure combination of > std::valarray and std::indirect_array but I don't want to bore you with all > the details. What it boils down to is this basic question about restrict: > > Given one restrict pointer based on another, should they never alias? > > As far as I understand the formal definition of "restrict" in section > 6.7.3.1 of the C standard [1], in the function below, pointer `y` is based > on "restrict" pointer `x`; hence, the compiler will assume that accesses *x > and *y might alias: > > void assign1(int *pA, long N) { > int *restrict x = pA; > { > int *y = x + N; > *x = *y; > } > } > > > However, what if y itself is declared "restrict": can the compiler assume > that *x and *y will never alias? > > void assign2(int *pA, long N) { > int *restrict x = pA; > { > int *restrict y = x + N; > *x = *y; > } > } > > Both Jeroen's and Hal's implementation (the intrinsic-based one) will say > "NoAlias" for the accesses in assign2() but shouldn't x and y be in the > same restrictness "bucket" since y is based on x? > > [1] http://port70.net/~nsz/c/c11/n1570.html#6.7.3.1 > <https://urldefense.proofpoint.com/v2/url?u=http-3A__port70.net_-7Ensz_c_c11_n1570.html-236.7.3.1&d=DwMFaQ&c=DPL6_X_6JkXFx7AXWqB0tg&r=ELyOnT0WepII6UnFk-OSzxlGOXXSfAvOLT6E8iPwwJk&m=xMDqkSAlj-YCOS4JMDXAENpBS-eaCcLYSkIm1qK68fs&s=B3LRzqpd9bD1724nvhG0FtpFh3QPsQ4FTBGQ4qJn1cA&e=> >-------------- next part -------------- An HTML attachment was scrubbed... URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200219/14510c24/attachment.html>
Apparently Analagous Threads
- Given one restrict pointer based on another, should they never alias?
- Adding meta-data when creating objects. e.g: changing "<-" so to (for example) add "creation time" - how-to and pros/cons?
- local restrict - again
- [LLVMdev] How to associate extra comments to a MachineInstruction ?
- Inlining + CSE + restrict pointers == funtimes