1 Calling Conventions Architecture Review: Caller- and Callee-Saved - - PowerPoint PPT Presentation

1

CS553 Lecture Register Allocation II 1

Prelude

– The Sears Tower contains enough concrete to build an eight-lane, five- mile-long highway, enough steel to build 50,000 automobiles, and enough telephone wiring to wrap around the world 1.75 times. – At the very top of the building, the maximum wind drift is just one foot. – Six roof-mounted robotic window-washing machines clean all 16,1000 windows on the Sears tower. – My son Kevin visited the “Serious” tower no less than 5 times during the two years we lived in Chicago.

CS553 Lecture Register Allocation II 2

Register Allocation II

– Register allocation – Global allocation via graph coloring

– More register allocation – Allocation across procedure calls

CS553 Lecture Register Allocation II 3

Interference Graph Allocators

Chaitin Briggs

CS553 Lecture Register Allocation II 4

Register Allocation and Procedure Calls

– Register values may change across procedure calls – The allocator must be sensitive to this

– Work within a well-defined calling convention – Use interprocedural allocation (not covering this)

2

CS553 Lecture Register Allocation II 5

Calling Conventions

– Fast calls (pass arguments in registers, minimal register saving/restoring) – Language-independent – Support debugging, profiling, garbage collection, etc.

– Varargs – Passing/returning aggregates – Exceptions, non-local returns – setjmp()/longjmp()

CS553 Lecture Register Allocation II 6

Architecture Review: Caller- and Callee-Saved Registers

– Caller-saved – Callee-saved

– Caller must save/restore these registers when live across call – Callee is free to use them

{ rcaller = 4 save rcaller goo() restore rcaller rcaller? } goo() { rcaller = 99 } goo() is free to modify rcaller caller callee

CS553 Lecture Register Allocation II 7

Architecture Review: Caller- and Callee-Saved Registers

– Callee must save/restore these registers when it uses them – Caller expects callee to not change them

foo() { rcallee = 4 goo() rcallee? } goo() { save rcallee rcallee = 99 restore rcallee } goo() promises not to modify rcallee caller callee

CS553 Lecture Register Allocation II 8

Architectures with Callee and Caller Registers

– hardware-saved %i0-%i7, %o0-%o8

– 7 callee-saved out of 32 registers

– caller-saved: $t0-$t9, $a0-$a3, $v0-$v1 – callee-saved: $s0-$s7, $ra, $fp

– 18 callee-saved – 14 caller-saved

– 4 callee-saved – 28 caller-saved

3

CS553 Lecture Register Allocation II 9

Register Allocation and Calling Conventions

– Save all live caller-saved registers before call; restore after – Save all used callee-saved registers at procedure entry; restore at return – Suboptimal

– Encode calling convention constraints in the IR and interference graph – How? A variable that is not live across calls should go in caller-saved registers A variable that is live across multiple calls should go in callee-saved registers

foo() { t = … … = t s = … f() g() … = s }

CS553 Lecture Register Allocation II 10

r1 r2 s1 f3 s2 s4 s3

floating point integer

Precolored Nodes

– Precolored (mutually interfering) – Not simplifiable – Not spillable

– Integers usually can’t be stored in floating point registers – Some instructions can only store result in certain registers – Caller-saved and callee-saved registers. . .

floating point integer

CS553 Lecture Register Allocation II 11

Precolored Nodes and Calling Conventions

– Treat entry as def of all callee-saved registers – Treat exit as use of them all – Allocator must “spill” callee-saved registers to use them

– Variables live across call interfere with all caller-saved registers

foo() { def(r3) use(r3) }

Live range of callee-saved registers

CS553 Lecture Register Allocation II 12

Example

foo(): def(r3) t1 := r3 a := ... b := ... ... a ... call goo ... b ... r3 := t1 use(r3) return

r1 r3 t1

b a r2

4

CS553 Lecture Register Allocation II 13

MiniJava compiler, Tiger pg. 198, Ch 11, Ch 12

– MipsFrame.java, calleeSaves = { RA, S0, S1, S2, S3, S4, S5, S6, S7}, – In MipsFrameRegAlloc.java procEntryExit1 – at entry, copy calleeSaves into temporaries – at end, copy from temporaries back into calleeSaves – In MipsFrameRegAlloc.java procEntryExit2 – at entry, could put a source statement with defs, but is not necessary – at end, already inserting a sink statement that uses all calleeSaves

– MipsFrame.java, callerSaves = {T0, T1, T2, T3, T4, T5, T6, T7, T8, T9, V0, V1}; – Put them all in destlist for the function call, see Codegen.java call to frame.callDefs() – Put use in sink statement so they are live from call to end of procedure

Special registers

– Cannot use for register allocation, might want to make FP (which is S8) special – In MipsFrame.java procEntryExit2 put uses of the special registers in sink stmt

Argument registers can also be used as caller-saved registers

– Codegen.java pushes all arguments onto the stack right now

CS553 Lecture Register Allocation II 14

Tradeoffs

+ Decreases code size: one procedure body may have multiple calls + Small procedures tend to need fewer registers than large ones; callee-save makes sense because procedure sizes are shrinking − May increase execution time: For long-lived variables, may save and restore registers multiple times, once for each procedure, instead of a single end-to-end save/restore

– We’re making local decisions for policies that require global information

CS553 Lecture Register Allocation II 15

Concepts

– Caller- vs. callee-saved registers – Precoloring

CS553 Lecture Register Allocation II 16

Next Time

Reading – Make sure to read the Briggs paper Lecture – Register Allocation III

5

CS553 Lecture Register Allocation II 17

Problem with Callee-Saved Registers

know register values in any stack frame – Caller-saved registers are on stack frame at known location – Callee-saved registers?

F2: save r1,r2 F4: save r3 F3: F1:

r1, r2 caller-saved r3 callee-saved