[cfe-dev] [OpenMP] Redundant store inside reduction loop body (original) (raw)

Itay Bookstein via cfe-dev cfe-dev at lists.llvm.org
Tue Dec 8 12:28:18 PST 2020

Hey,

I've encountered a peculiar code generation issue in a parallel-for-reduction. Inside the per-thread reduction loop, I see a store to a stack slot that happens every iteration, clobbering the value written by the previous one. The address of that stack slot is later taken to pass to the inter-thread reduction, but the store is not hoisted outside the loop, while I'd expect it to happen just once outside it.

Attached below are the code example that triggers this, and an annotated x86-64 assembly snippet from the outlined OMP function. The code was compiled using clang++-12 Ubuntu focal unstable branch, using the command-line: clang++-12 -fopenmp -O3 main.cpp -o main

I'm wondering whether this is some sort of bug, and in which component.

Regards, ~Itay

// main.cpp #include #include

double compute_dot_product(size_t n, double *xv, double *yv) { double local = 0.0; #pragma omp parallel for reduction (+:local) for (size_t i = 0; i < n; i++) local += xv[i] * yv[i]; return local; }

int main(int argc, char **argv) { constexpr size_t n = 0x1000; auto xv = std::make_unique<double[]>(n); auto yv = std::make_unique<double[]>(n);

double result = compute_dot_product(n, xv.get(), yv.get()); printf("result = %e\n", result); return 0; }

// Disassembly excerpt from objdump -d --no-show-raw-insn main, function <...>omp_outlined<...> 4012f0: movsd (%rdx,%rcx,8),%xmm1 ; <--- Non-unrolled loop head 4012f5: mulsd (%rsi,%rcx,8),%xmm1 4012fa: addsd %xmm1,%xmm0 4012fe: movsd %xmm0,(%rsp) ; <--- Un-hoisted store every loop iteration 401303: add $0x1,%rcx 401307: add $0xffffffffffffffff,%rax 40130b: jne 4012f0 <.omp_outlined.+0xc0> ; <--- Non-unrolled loop latch 40130d: cmp $0x3,%rbp 401311: jb 40138b <.omp_outlined.+0x15b> ; <--- x4-unrolled loop guard 401313: sub %rcx,%rdi ; <--- x4-unrolled loop preheader 401316: lea (%rsi,%rcx,8),%rsi 40131a: add $0x18,%rsi 40131e: lea (%rdx,%rcx,8),%rcx 401322: add $0x18,%rcx 401326: mov $0xffffffffffffffff,%rdx 40132d: nopl (%rax) 401330: movsd -0x10(%rcx,%rdx,8),%xmm1 ; <--- x4-unrolled loop head 401336: mulsd -0x10(%rsi,%rdx,8),%xmm1 40133c: addsd %xmm0,%xmm1 401340: movsd %xmm1,(%rsp) ; <--- Weird store #1 401345: movsd -0x8(%rcx,%rdx,8),%xmm0 40134b: mulsd -0x8(%rsi,%rdx,8),%xmm0 401351: addsd %xmm1,%xmm0 401355: movsd %xmm0,(%rsp) ; <--- Weird store #2 40135a: movsd (%rcx,%rdx,8),%xmm1 40135f: mulsd (%rsi,%rdx,8),%xmm1 401364: addsd %xmm0,%xmm1 401368: movsd %xmm1,(%rsp) ; <--- Weird store #3 40136d: movsd 0x8(%rcx,%rdx,8),%xmm0 401373: mulsd 0x8(%rsi,%rdx,8),%xmm0 401379: addsd %xmm1,%xmm0 40137d: movsd %xmm0,(%rsp) ; <--- Weird store #4 401382: add $0x4,%rdx 401386: cmp %rdx,%rdi 401389: jne 401330 <.omp_outlined.+0x100> ; <--- x4-unrolled loop latch 40138b: mov $0x402028,%edi 401390: mov %r14d,%esi 401393: callq 401040 <__kmpc_for_static_fini at plt> 401398: mov %rsp,%rax ; <--- Load address of the stack slot to pass to reduction logic 40139b: mov %rax,0x20(%rsp)

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