| Issue |
152639
|
| Summary |
[Flang][OpenMP] Heap allocation for private variables prevents SROA optimization
|
| Labels |
llvm:optimizations,
flang,
missed-optimization,
flang:openmp
|
| Assignees |
|
| Reporter |
yus3710-fj
|
I found that 535.weather_t in SPEChpc 2021 experiences an 8.9% slowdown on NVIDIA Grace after #125732. This is because one of the loops is no longer vectorized. If OpenMP is disabled, the loop is still vectorized because `memcpy` is not called within the loop. (In addition, OpenMP with `-mllvm -disable-loop-idiom-memcpy` flag also helps vectorization.)
reproducer: https://godbolt.org/z/5TYohj1d6
Before the patch, the array assignment for `stencil` was replaced with `memcpy` in LoopIdiomRecognize once, and then both `stencil` and `memcpy` were removed in SROA. We can remove `stencil` here because it is just an alias of `in3d(i-2:i+1, k)`. After the patch, the optimization in SROA doesn't work, and `memcpy` remains in the loop. It seems that the private variable of `stencil` is allocated not on the stack but on the heap, which prevents the optimization.
The following is LLVM IR before SROA.
* Before the patch
```llvm
define internal void @_QMreproPsub..omp_par(ptr noalias readnone captures(none) %tid.addr, ptr noalias readnone captures(none) %zero.addr, ptr readonly captures(none) %0) #0 {
omp.par.entry:
%loadgep_.reloaded = load ptr, ptr %0, align 8
%gep_.reloaded12 = getelementptr i8, ptr %0, i64 8
%loadgep_.reloaded12 = load ptr, ptr %gep_.reloaded12, align 8
%gep_ = getelementptr i8, ptr %0, i64 16
%loadgep_ = load ptr, ptr %gep_, align 8
%gep_1 = getelementptr i8, ptr %0, i64 24
%loadgep_2 = load ptr, ptr %gep_1, align 8
%p.lastiter = alloca i32, align 4
%p.lowerbound = alloca i32, align 4
%p.upperbound = alloca i32, align 4
%p.stride = alloca i32, align 4
%1 = load i64, ptr %loadgep_.reloaded, align 8
%2 = load i64, ptr %loadgep_.reloaded12, align 8
%3 = alloca [4 x double], align 8
%4 = alloca [4 x double], align 8 ;; stencil(private)
...
omp.wsloop.region6.preheader: ; preds = %omp_loop.body
%scevgep = getelementptr i8, ptr %loadgep_2, i64 %25
call void @llvm.memcpy.p0.p0.i64(ptr align 8 %4, ptr align 8 %scevgep, i64 32, i1 false), !tbaa !9 ;; stencil(:) = in3d(i-2:i+1, k)
br label %omp.wsloop.region8
```
* After the patch
```llvm
define internal void @_QMreproPsub..omp_par(ptr noalias readnone captures(none) %tid.addr, ptr noalias readnone captures(none) %zero.addr, ptr readonly captures(none) %0) #2 {
omp.par.entry:
%loadgep_.reloaded = load ptr, ptr %0, align 8
%gep_.reloaded23 = getelementptr i8, ptr %0, i64 8
%loadgep_.reloaded23 = load ptr, ptr %gep_.reloaded23, align 8
%gep_3 = getelementptr i8, ptr %0, i64 32
%loadgep_4 = load ptr, ptr %gep_3, align 8
%gep_5 = getelementptr i8, ptr %0, i64 40
%loadgep_6 = load ptr, ptr %gep_5, align 8
%p.lastiter = alloca i32, align 4
%p.lowerbound = alloca i32, align 4
%p.upperbound = alloca i32, align 4
%p.stride = alloca i32, align 4
%1 = load i64, ptr %loadgep_.reloaded, align 8
%2 = load i64, ptr %loadgep_.reloaded23, align 8
%3 = load i32, ptr @_QMreproEnx, align 4, !tbaa !3
%4 = tail call dereferenceable_or_null(32) ptr @malloc(i64 32) ;; stencil(private)
...
omp.wsloop.region10.preheader: ; preds = %omp_loop.body
%scevgep = getelementptr i8, ptr %loadgep_6, i64 %25
call void @llvm.memcpy.p0.p0.i64(ptr align 8 %4, ptr align 8 %scevgep, i64 32, i1 false), !tbaa !9 ;; stencil(:) = in3d(i-2:i+1, k)
br label %omp.wsloop.region12
```
And we can get the following IR by SROA.
* Before the patch
```llvm
;; only loads in3d(i-2:i+1, k)
omp.wsloop.region6.preheader: ; preds = %omp_loop.body
%scevgep = getelementptr i8, ptr %loadgep_2, i64 %17
%.sroa.0.0.copyload = load double, ptr %scevgep, align 8, !tbaa !9
%.sroa.8.0.scevgep.sroa_idx = getelementptr inbounds i8, ptr %scevgep, i64 8
%.sroa.8.0.copyload = load double, ptr %.sroa.8.0.scevgep.sroa_idx, align 8, !tbaa !9
%.sroa.12.0.scevgep.sroa_idx = getelementptr inbounds i8, ptr %scevgep, i64 16
%.sroa.12.0.copyload = load double, ptr %.sroa.12.0.scevgep.sroa_idx, align 8, !tbaa !9
%.sroa.16.0.scevgep.sroa_idx = getelementptr inbounds i8, ptr %scevgep, i64 24
%.sroa.16.0.copyload = load double, ptr %.sroa.16.0.scevgep.sroa_idx, align 8, !tbaa !9
br label %omp.wsloop.region8
```
* After the patch
```llvm
;; no difference
omp.wsloop.region10.preheader: ; preds = %omp_loop.body
%scevgep = getelementptr i8, ptr %loadgep_6, i64 %25
call void @llvm.memcpy.p0.p0.i64(ptr align 8 %4, ptr align 8 %scevgep, i64 32, i1 false), !tbaa !9 ;; stencil(:) = in3d(i-2:i+1, k)
br label %omp.wsloop.region12
```
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