[Patch] Teach InstCombine to work with smaller legal types in icmp (shl %v, C1), C2

[Arnaud A. de Grandmaison]

This patch teaches InstCombine to work with smaller legal types in icmp
(shl %v, C1), C2.

Using smaller constants is target friendly, at least for those which can
compare to immediate. It also avoids inserting a shift in favor of a
trunc, which can be free on those targets where we are using a legal type.

This used to work up to LLVM-3.1, but regressed with the 3.2 release.

This pattern can occur frequently because of the integer promotion rules
in C/C++ code, which will generate "icmp (sext(trunc %v)), constant".
With 3.2, the sext(trunc) is combined to shifts by visitSExt, before the
icmp combining has a chance to kick in. When it kicks in, it faces a
pattern which is not recognized.

Any comments or ok to commit ?

I found it with the following testcase :

$ cat testcase.c
#include <stdint.h>

uint16_t
sw_modulo (uint16_t val)
{
  uint16_t start = 0;
  uint16_t end = 349;
  int16_t res = 0;

  do {  
    uint16_t cpt_i = (start + end)/2;
    res = val - cpt_i*36;

    if (res < 0)
      end = cpt_i - 1;
    else
      start = cpt_i + 1;

  } while ( (res >= 36) || (res < 0) );

  return res;
}


Without the patch, the X86_64 loop code looks like :
...
.LBB0_2:                                # %.thread
    addl    $65535, %edx            # imm = 0xFFFF
    movzwl    %dx, %r8d
    .align    16, 0x90
.LBB0_1:                                # %.critedge
    movzwl    %cx, %edx
    addl    %r8d, %edx
    shrl    %edx
    movzwl    %dx, %eax
    imull    $-36, %eax, %eax
    addl    %edi, %eax
    movl    %eax, %esi
    shll    $16, %esi
    testl    %esi, %esi
    js    .LBB0_2
    incl    %edx
    cmpl    $2293760, %esi          # imm = 0x230000
    movw    %dx, %cx
    ja    .LBB0_1
...

With the patch:
...
.LBB0_2:                                # %.thread
    addl    $65535, %edx            # imm = 0xFFFF
    movzwl    %dx, %eax
    .align    16, 0x90
.LBB0_1:                                # %.critedge
    movzwl    %cx, %edx
    addl    %eax, %edx
    shrl    %edx
    imull    $-36, %edx, %esi
    addl    %edi, %esi
    testw    %si, %si
    js    .LBB0_2
    incl    %edx
    cmpw    $35, %si
    movw    %dx, %cx
    ja    .LBB0_1
...

Cheers,

-- 
Arnaud A. de Grandmaison

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