Code Generation Questions and Answers

Q: State whether the following is true or false.A code generator may assume the target machine has an unlimited supply of registers.

Correct answer: FalseThe machine has whatever registers it has, and running out is the central difficulty of the back end. An intermediate representation may assume unlimited virtual registers, but the code generator must map them onto the real set and spill to memory where it cannot - which is exactly what register allocation is.

51.

A frame pointer is sometimes omitted from generated code in order to

A.reduce the number of instructions in the procedure body
B.allow the procedure to call itself recursively
C.free one more register for allocation to values
D.make the generated code easier for a debugger to read

Answer: C

Correct answer: free one more register for allocation to values

The frame pointer occupies a register solely to make the frame layout easy to follow, and where the frame size is known at compile time the stack pointer alone will serve. Omitting it recovers a register, which on a machine with few of them is worth having - at the cost of making stack traces harder to reconstruct.

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52.

Generating code for an array reference requires

A.one instruction that names the array and the subscript together
B.the subscript to be a constant known before the program runs
C.the whole array to be loaded into registers first
D.multiplying the index by the element width and adding the base

Answer: D

Machines with a scaled index addressing mode can perform the computation as part of the access. Where such a mode is absent the generator must emit explicit arithmetic. Strength reduction can often replace the multiplication with an addition.

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53.

A machine offering a scaled indexed addressing mode allows the code generator to

A.fold the array address computation into the access instruction
B.dispense with registers for array accesses altogether
C.avoid checking the array bounds at run time
D.store arrays in a different format from other data

Answer: A

One instruction then performs the multiplication, the addition and the access. Exploiting such modes is a large part of what makes instruction selection worth doing carefully. A generator that ignores them emits three instructions where one would serve.

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54.

State whether the following is true or false.
A code generator may assume the target machine has an unlimited supply of registers.

A.True
B.False
C.True only at large scales
D.False only at small scales

Answer: B

Correct answer: False

The machine has whatever registers it has, and running out is the central difficulty of the back end. An intermediate representation may assume unlimited virtual registers, but the code generator must map them onto the real set and spill to memory where it cannot - which is exactly what register allocation is.

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55.

Virtual registers in an intermediate representation are

A.memory locations reserved for the compiler's own use
B.registers that exist only while the compiler itself is running
C.an unlimited supply of names later mapped onto real registers
D.the registers the target machine actually provides

Answer: C

Generating code against an unlimited set separates selection from allocation. The allocator afterwards maps them onto the real set, spilling wherever the mapping does not fit. Separating the two concerns makes each of them considerably simpler to write.

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56.

Generating code for a conditional statement usually produces

A.a single instruction that evaluates the whole statement
B.a jump table indexed by the value of the condition
C.no instructions at all when the condition is a variable
D.a comparison, a conditional jump and one or two labels

Answer: D

The comparison sets condition codes or produces a boolean, and the jump acts on it. Which form is used depends on how the target machine expresses comparisons. Some machines produce a boolean in a register instead of setting flags.

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57.

A machine that sets condition codes as a side effect of arithmetic allows the generator to

A.omit a separate comparison when the value was just computed
B.dispense with conditional jumps altogether in the generated code
C.evaluate both branches of a conditional simultaneously
D.avoid allocating any registers for boolean values

Answer: A

If the flags already reflect the value, the comparison is redundant. Tracking the state of the condition codes is a small but worthwhile part of a good back end. The saving is one instruction per comparison, which adds up inside a loop.

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58.

Generating code for a loop benefits from

A.placing the loop body in a separate procedure
B.keeping the loop control variable in a register
C.storing the loop variable to memory on each iteration
D.recomputing the loop bound at the start of every iteration

Answer: B

Correct answer: keeping the loop control variable in a register

The control variable is read and written on every iteration, so it is the value that most repays a register. Storing it to memory each time round would add two memory references per iteration, and recomputing the bound would add more - both being work the loop multiplies.

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59.

Assertion (A): Register allocators weight uses inside loops more heavily than uses outside them.
Reason (R): An instruction inside a loop executes many times, so keeping its operands in registers saves proportionately more.

A.A is true but R is false
B.A is false but R is true
C.Both A and R are true, and R is the correct explanation of A
D.Both A and R are true, but R is not the correct explanation of A

Answer: C

Correct answer: Both A and R are true, and R is the correct explanation of A

Both are true and the reason gives the mechanism. A use inside a loop costs whatever it costs multiplied by the trip count, so the allocator weights it accordingly - commonly by ten per level of nesting. That weighting is what makes the allocator spill the outer values and keep the inner ones.

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60.

A use inside a loop nested two deep is weighted by a factor of ten per level. What weight does a single use there receive?

A.20
B.10
C.2
D.100

Answer: D

Two levels of nesting give ten times ten, that is one hundred. The heuristic is crude but reflects the fact that inner loop code dominates execution time in most programs. Profile data replaces the heuristic with a measurement when it is available.

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