Fwd: Fix Logical Right Shift Simplify Bug:

[林作健 via llvm-commits]

Problem code:
bool fits_shifter(uint32_t imm32,
                         uint32_t* rotate_imm,
                         uint32_t* immed_8,
                         Instr* instr) {
  // imm32 must be unsigned.
  for (int rot = 0; rot < 16; rot++) {
    uint32_t imm8 = (imm32 << 2*rot) | (imm32 >> (32 - 2*rot));
    if ((imm8 <= 0xff)) {
      *rotate_imm = rot;
      *immed_8 = imm8;
      return true;
    }
  }
  return false;
}

After loop unroll pass, which has never been enabled before 3.8, the llvm
IR:
for.body:                                         ; preds = %tailrecurse
  br i1 false, label %cleanup3, label %for.inc

for.inc:                                          ; preds = %for.body
  %shl.1 = shl i32 %imm32.tr, 2
  %shr.1 = lshr i32 %imm32.tr, 30
  %or.1 = or i32 %shr.1, %shl.1
  %cmp2.1 = icmp ult i32 %or.1, 256
  br i1 %cmp2.1, label %cleanup3, label %for.inc.1

As you see, the for.body block becomes a trivial block, which
is caused by instruction simplify, like the following:

for.body:                                         ; preds = %tailrecurse
  %mul = shl nsw i32 0, 1
  %shl = shl i32 %imm32.tr, %mul
  %sub = sub nuw nsw i32 32, %mul
  %shr = lshr i32 %imm32.tr, %sub
  %or = or i32 %shr, %shl
  %cmp2 = icmp ult i32 %or, 256
  br i1 %cmp2, label %cleanup3, label %for.inc


When simplifying  %shr = lshr i32 %imm32.tr, %sub with %sub is a constant
32,
the value undefined will be generated. Then %or = or i32 %shr, %shl will
generate a
constant -1, and then cause the whole block trivially branch to the
negative branch.

My patch in the attachment tries to fix this by generating constant 0 when
the right shift exceeds
the types bit limits. And That's what is expecting in the real life
scenario.
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