On 2016-08-26 11:32, Mehdi Amini wrote:> Hi, > >> Recently, I noticed that less number of global variables are merged in >> global-merge pass and in some global variable, array types are used >> instead of its original type. For example, [4xi8] with align 4 is used >> for a i32 global variable. For me, it seems that such pattern is >> observed after r278338 (Resolution-based LTO API). > > Are you sure it is performed in the global merge pass? Can you provide > an example of input IR where you see this now but didn’t before? > Also can you confirm you’re using the gold-linker?I used gold linker. In spec2006/perlbench, I observed the less number of globals are merged in GlobalMerge.cpp after r278338. The reason is because, from the very first pass, several global variables use [4xi8] with align 4, instead of its original type i32 after r278338. Current GlobalMerge pass doesn't seem to handle such fancy-aligned globals. If such type change (e.g., from i32 to [4xi8]) in global variables was intended in r278338, I think we should enhance GlobalMerge to handle such cases.> >> Can anyone give me little bit more insight if such type change was >> intended? If then why such uses of array type is needed? > > The transformation I know of is the common variables merging, bug it > is not new. We use an array because we have to merge globals from > different size and an array of bytes is the most straightforward > representation for that. > > — > Mehdi-- Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.
Mehdi Amini via llvm-dev
2016-Aug-26 16:47 UTC
[llvm-dev] Use of array type in globals in LTO
> On Aug 26, 2016, at 9:06 AM, junbuml at codeaurora.org wrote: > > On 2016-08-26 11:32, Mehdi Amini wrote: >> Hi, >>> Recently, I noticed that less number of global variables are merged in global-merge pass and in some global variable, array types are used instead of its original type. For example, [4xi8] with align 4 is used for a i32 global variable. For me, it seems that such pattern is observed after r278338 (Resolution-based LTO API). >> Are you sure it is performed in the global merge pass? Can you provide >> an example of input IR where you see this now but didn’t before? >> Also can you confirm you’re using the gold-linker? > > I used gold linker. In spec2006/perlbench, I observed the less number of globals are merged in GlobalMerge.cpp after r278338. The reason is because, from the very first pass, several global variables use [4xi8] with align 4, instead of its original type i32 after r278338. Current GlobalMerge pass doesn't seem to handle such fancy-aligned globals. If such type change (e.g., from i32 to [4xi8]) in global variables was intended in r278338, I think we should enhance GlobalMerge to handle such cases.Can you submit a reproduction for Gold please? We need to understand what changed with the new LTO API. — Mehdi
On 2016-08-26 12:47, Mehdi Amini wrote:>> On Aug 26, 2016, at 9:06 AM, junbuml at codeaurora.org wrote: >> >> On 2016-08-26 11:32, Mehdi Amini wrote: >>> Hi, >>>> Recently, I noticed that less number of global variables are merged >>>> in global-merge pass and in some global variable, array types are >>>> used instead of its original type. For example, [4xi8] with align 4 >>>> is used for a i32 global variable. For me, it seems that such >>>> pattern is observed after r278338 (Resolution-based LTO API). >>> Are you sure it is performed in the global merge pass? Can you >>> provide >>> an example of input IR where you see this now but didn’t before? >>> Also can you confirm you’re using the gold-linker? >> >> I used gold linker. In spec2006/perlbench, I observed the less number >> of globals are merged in GlobalMerge.cpp after r278338. The reason is >> because, from the very first pass, several global variables use [4xi8] >> with align 4, instead of its original type i32 after r278338. Current >> GlobalMerge pass doesn't seem to handle such fancy-aligned globals. If >> such type change (e.g., from i32 to [4xi8]) in global variables was >> intended in r278338, I think we should enhance GlobalMerge to handle >> such cases. > > Can you submit a reproduction for Gold please? > We need to understand what changed with the new LTO API.I compiled below C code for aarch64 in lto using gold (--target=aarch64-linux-gnu -flto -fuse-ld=gold). After r278338, two globals, GVi32_a and GVi32_b, are [4 x i8] type in the input IR to GlobalMerge. Therefore, GlobalMerge do not even start to handle them because as of now it ignores fancy-aligned globals. Before r278338, GVi32_a and GVi32_b seems to be i32 in the input IR to GlobalMerge. Is this change in the input IR expected ? ------------------------- int GVi32_a ; int GVi32_b ; __attribute__((noinline)) void setGV(int a) { GVi32_a = a ; GVi32_b = a ; } __attribute__((noinline)) int loadGV() { return GVi32_a + GVi32_b ; } int main(int argc, char *argv[]){ setGV(argc); return loadGV(); } -------------------------> > > > — > Mehdi-- Qualcomm Datacenter Technologies, Inc. as an affiliate of Qualcomm Technologies, Inc. Qualcomm Technologies, Inc. is a member of the Code Aurora Forum, a Linux Foundation Collaborative Project.