[LLVMbugs] [Bug 22230] New: LLVM does a poor job of rematerializing address offsets which will fold into the addressing mode

[bugzilla-daemon at llvm.org]

http://llvm.org/bugs/show_bug.cgi?id=22230

            Bug ID: 22230
           Summary: LLVM does a poor job of rematerializing address
                    offsets which will fold into the addressing mode
           Product: libraries
           Version: trunk
          Hardware: PC
                OS: All
            Status: NEW
          Severity: normal
          Priority: P
         Component: Common Code Generator Code
          Assignee: unassignedbugs at nondot.org
          Reporter: djasper at google.com
                CC: llvmbugs at cs.uiuc.edu
    Classification: Unclassified

A certain code pattern, where a function mostly consists of a large loop around
a switch or if-else condition results in large live ranges for basic address
computations. The register allocator fails to rematerialize these address
computations in addressing modes of memory operands and instead spills and
reloads them from the stack dramatically increasing stack usage and generally
causing badness.

Small reproduction:

struct A {
  unsigned a;
  unsigned b;
  unsigned c;
  unsigned d;
};

void assign(unsigned *val);

void f(unsigned char *input, A *a) {
  for (;;) {
    unsigned char t = (++input)[0];
    if (t == 0) {
      assign(&a->a);
    } else if (t == 1) {
      assign(&a->b);
    } else if (t == 2) {
      assign(&a->c);
    } else if (t == 3) {
      assign(&a->d);
    }
  }
}

Resulting in:

__Z1fPhP1A:                             ## @_Z1fPhP1A
    .cfi_startproc
## BB#0:                                ## %entry
    pushq    %rbp
Ltmp0:
    .cfi_def_cfa_offset 16
Ltmp1:
    .cfi_offset %rbp, -16
    movq    %rsp, %rbp
Ltmp2:
    .cfi_def_cfa_register %rbp
    pushq    %r15
    pushq    %r14
    pushq    %r13
    pushq    %r12
    pushq    %rbx
    subq    $24, %rsp
Ltmp3:
    .cfi_offset %rbx, -56
Ltmp4:
    .cfi_offset %r12, -48
Ltmp5:
    .cfi_offset %r13, -40
Ltmp6:
    .cfi_offset %r14, -32
Ltmp7:
    .cfi_offset %r15, -24
    movq    %rdi, %rbx
    leaq    4(%rsi), %rax
    movq    %rax, -48(%rbp)         ## 8-byte Spill
    leaq    8(%rsi), %rax
    movq    %rax, -56(%rbp)         ## 8-byte Spill
    leaq    12(%rsi), %rax
    movq    %rax, -64(%rbp)         ## 8-byte Spill
    leaq    16(%rsi), %r14
    leaq    20(%rsi), %r15
    movq    %rsi, %r13
    incq    %rbx
    leaq    LJTI0_0(%rip), %r12
    jmp    LBB0_1
[.....]

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