[llvm-bugs] [Bug 30729] New: [PPC] Constant loads not hoisted out of loop

[via llvm-bugs]

https://llvm.org/bugs/show_bug.cgi?id=30729

            Bug ID: 30729
           Summary: [PPC] Constant loads not hoisted out of loop
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: Scalar Optimizations
          Assignee: unassignedbugs at nondot.org
          Reporter: echristo at gmail.com
                CC: llvm-bugs at lists.llvm.org
    Classification: Unclassified

In f(), LLVM hoists the loads for constants k1 and k2 outside loop.  In both
g() and h(), the loads stay inside, resulting in a longer loop body. Both g()
and h() have an if-statement enclosing the loads, so always loading the
constants is not necessarily better, since it is possible that the if-condition
may not be true.  However, when adding __builtin_expect in h() to hint the
compiler that the loads are very likely to be executed. In that case, I'd
probably expect the compiler to move the loads out of the loop body.

---
#include <altivec.h>
#include <stdint.h>

struct S {
  vector int k1;
  vector int k2;

  vector int f(uint64_t n, vector int x) const;
  vector int g(uint64_t n, vector int x) const;
  vector int h(uint64_t n, vector int x) const;
};

vector int S::f(uint64_t n, vector int x) const {
  const uint64_t base = 16;
  for ( ; n ; n /= base) {
    x += vec_ld(0, &k1);
    x ^= vec_ld(0, &k2);
  }
  return x;
}

vector int S::g(uint64_t n, vector int x) const {
  const uint64_t base = 16;
  for ( ; n ; n /= base) {
    const uint64_t d = n % base;
    if (d != 0) {
      x += vec_ld(0, &k1);
      x ^= vec_ld(0, &k2);
    }
  }
  return x;
}

vector int S::h(uint64_t n, vector int x) const {
  const uint64_t base = 16;
  for ( ; n ; n /= base) {
    const uint64_t d = n % base;
    if (__builtin_expect(d != 0, 1)) {
      x += vec_ld(0, &k1);
      x ^= vec_ld(0, &k2);
    }
  }
  return x;
}
---

        .text
        .abiversion 2
        .file   "constants.cc"
        .globl  _ZNK1S1fEyDv4_i
        .p2align        4
        .type   _ZNK1S1fEyDv4_i, at function
_ZNK1S1fEyDv4_i:                        # @_ZNK1S1fEyDv4_i
.Lfunc_begin0:
# BB#0:
        cmpldi   4, 0
        beq      0, .LBB0_4
# BB#1:
        li 5, 16
        lvx 3, 0, 3
        xxlor 34, 34, 34
        lvx 4, 3, 5
        .p2align        4
.LBB0_2:                                # =>This Inner Loop Header: Depth=1
        vadduwm 2, 3, 2
        rldicl. 4, 4, 60, 4
        xxlxor 34, 34, 36
        bne      0, .LBB0_2
# BB#3:
                                        # kill: %V2<def> %V2<kill> %VSH2<kill>
        blr
.LBB0_4:
        xxlor 34, 34, 34
                                        # kill: %V2<def> %V2<kill> %VSH2<kill>
        blr
        .long   0
        .quad   0
.Lfunc_end0:
        .size   _ZNK1S1fEyDv4_i, .Lfunc_end0-.Lfunc_begin0

        .globl  _ZNK1S1gEyDv4_i
        .p2align        4
        .type   _ZNK1S1gEyDv4_i, at function
_ZNK1S1gEyDv4_i:                        # @_ZNK1S1gEyDv4_i
.Lfunc_begin1:
# BB#0:
        cmpldi   4, 0
        beq      0, .LBB1_6
# BB#1:
        xxlor 34, 34, 34
        addi 5, 3, 16
        .p2align        5
.LBB1_2:                                # =>This Inner Loop Header: Depth=1
        rldicl. 6, 4, 0, 60
        beq      0, .LBB1_4
# BB#3:                                 #   in Loop: Header=BB1_2 Depth=1
        lvx 3, 0, 3
        lvx 4, 0, 5
        vadduwm 2, 3, 2
        xxlxor 34, 34, 36
.LBB1_4:                                #   in Loop: Header=BB1_2 Depth=1
        rldicl. 4, 4, 60, 4
        bne      0, .LBB1_2
# BB#5:
                                        # kill: %V2<def> %V2<kill> %VSH2<kill>
        blr
.LBB1_6:
        xxlor 34, 34, 34
                                        # kill: %V2<def> %V2<kill> %VSH2<kill>
        blr
        .long   0
        .quad   0
.Lfunc_end1:
        .size   _ZNK1S1gEyDv4_i, .Lfunc_end1-.Lfunc_begin1

        .globl  _ZNK1S1hEyDv4_i
        .p2align        4
        .type   _ZNK1S1hEyDv4_i, at function
_ZNK1S1hEyDv4_i:                        # @_ZNK1S1hEyDv4_i
.Lfunc_begin2:
# BB#0:
        cmpldi   4, 0
        beq      0, .LBB2_6
# BB#1:
        xxlor 34, 34, 34
        addi 5, 3, 16
        .p2align        5
.LBB2_2:                                # =>This Inner Loop Header: Depth=1
        rldicl. 6, 4, 0, 60
        beq      0, .LBB2_4
# BB#3:                                 #   in Loop: Header=BB2_2 Depth=1
        lvx 3, 0, 3
        lvx 4, 0, 5
        vadduwm 2, 3, 2
        xxlxor 34, 34, 36
.LBB2_4:                                #   in Loop: Header=BB2_2 Depth=1
        rldicl. 4, 4, 60, 4
        bne      0, .LBB2_2
# BB#5:
                                        # kill: %V2<def> %V2<kill> %VSH2<kill>
        blr
.LBB2_6:
        xxlor 34, 34, 34
                                        # kill: %V2<def> %V2<kill> %VSH2<kill>
        blr
        .long   0
        .quad   0
.Lfunc_end2:
        .size   _ZNK1S1hEyDv4_i, .Lfunc_end2-.Lfunc_begin2

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