1. Building SSA Form COMP 512 Rice University Houston, Texas Fall 2003
Copyright 2003, Keith D. Cooper & Linda Torczon, all rights reserved.
Students enrolled in Comp 512 at Rice University have explicit permission to make copies of these materials for their personal use.

2. Review SSA-form Each name is defined exactly once
Each use refers to exactly one name
What’s hard
Straight-line code is trivial
Splits in the CFG are trivial
Joins in the CFG are hard
Building SSA Form
Insert Ø-functions at birth points of values
Rename all values for uniqueness
A Ø-function is a special kind of copy that selects one of its parameters.
The choice of parameter is governed by the CFG edge along which control reached the current block.
Few machines implement a Ø-function directly in hardware.
y1  ...
y2  ...
y3  Ø(y1,y2)
COMP 512, Fall 2003 *

3. SSA Construction Algorithm (High-level sketch)
1. Insert Ø-functions
2. Rename values
… that’s all ...
… of course, there is some bookkeeping to be done ...
COMP 512, Fall 2003 *

4. SSA Construction Algorithm (Less high-level sketch)
1. Insert Ø-functions at every join for every name
2. Solve reaching definitions
3. Rename each use to the def that reaches it (will be unique)
What’s wrong with this approach
Too many Ø-functions (precision)
Too many Ø-functions (space)
Too many Ø-functions (time)
Need to relate edges to Ø-functions parameters (bookkeeping)
To do better, we need a more complex approach
Builds maximal SSA
COMP 512, Fall 2003

5. SSA Construction Algorithm (Less high-level sketch)
1. Insert Ø-functions
a.) calculate dominance frontiers
b.) find global names
for each name, build a list of blocks that define it
c.) insert Ø-functions
 global name n
 block b in which n is assigned
 block d in b’s dominance frontier
insert a Ø-function for n in d
add d to n’s list of defining blocks
Moderately complex
Compute list of blocks where each name is assigned & use as a worklist
This adds to the worklist ! {
Creates the iterated dominance frontier
Use a checklist to avoid putting blocks on the worklist twice; keep another checklist to avoid inserting the same Ø-function twice.
COMP 512, Fall 2003 *

6. SSA Construction Algorithm (Less high-level sketch)
2. Rename variables in a pre-order walk over dominator tree
(use an array of stacks, one stack per global name)
Staring with the root block, b
a.) generate unique names for each Ø-function
and push them on the appropriate stacks
b.) rewrite each operation in the block
i. Rewrite uses of global names with the current version
(from the stack)
ii. Rewrite definition by inventing & pushing new name
c.) fill in Ø-function parameters of successor blocks
d.) recurse on b’s children in the dominator tree
e.) <on exit from block b > pop names generated in b from stacks
1 counter per name for subscripts
Reset the state
Need the end-of-block name for this path
COMP 512, Fall 2003 *

7. SSA Construction Algorithm (Low-level detail)
Computing Dominance
First step in Ø-function insertion computes dominance
Recall that n dominates m iff n is on every path from n0 to m
Every node dominates itself
n’s immediate dominator is its closest dominator, IDOM(n)†
DOM(n0 ) = { n0 }
DOM(n) = { n }  (ppreds(n) DOM(p))
Computing DOM
These equations form a rapid data-flow framework
Iterative algorithm will solve them in d(G) + 3 passes
Each pass does N unions & E intersections,
E is O(N 2),  O(N 2) work
Initially, DOM(n) = N,  n≠n0
†IDOM(n ) ≠ n, unless n is n0, by convention.
COMP 512, Fall 2003

8. Example Progress of iterative solution for DOM B1 B2 B3 B4 B5 B6 B7 B0
Results of iterative solution for DOM
Flow Graph
COMP 512, Fall 2003 *

9. Example Progress of iterative solution for DOM B1 B2 B3 B4 B5 B6 B7 B0
Results of iterative solution for DOM
Dominance Tree
There are asymptotically faster algorithms.
With the right data structures, the iterative algorithm can be made faster.
See Cooper, Harvey, & Kennedy, on the web site.
COMP 512, Fall 2003

10. Example Dominance Frontiers & Ø-Function Insertion
A definition at n forces a Ø-function at m iff
n  DOM(m) but n  DOM(p) for some p  preds(m)
DF(n ) is fringe just beyond region n dominates B1 B2 B3 B4 B5 B6 B7 B0
x Ø(...)
 in 7 forces Ø-function in DF(7) = {1}
x ...
x Ø(...)
DF(4) is {6}, so  in 4 forces Ø-function in 6
x Ø(...)
 in 6 forces Ø-function in DF(6) = {7}
Dominance Frontiers
 in 1 forces Ø-function in DF(1) = Ø (halt )
For each assignment, we insert the Ø-functions
COMP 512, Fall 2003 *

11. Example Computing Dominance Frontiers
Only join points are in DF(n) for some n
Leads to a simple, intuitive algorithm for computing dominance frontiers
For each join point x (i.e., |preds(x)| > 1)
For each CFG predecessor of x
Run up to IDOM(x ) in the dominator tree, adding
x to DF(n) for each n between x and IDOM(x ) B1 B2 B3 B4 B5 B6 B7 B0
For some applications, we need post-dominance, the post-dominator tree, and reverse dominance frontiers, RDF(n )
Just dominance on the reverse CFG
Reverse the edges & add unique exit node
We will use these in dead code elimination
Dominance Frontiers
COMP 512, Fall 2003 *

12. SSA Construction Algorithm (Reminder)
1. Insert Ø-functions at every join for every name
a.) calculate dominance frontiers
b.) find global names
for each name, build a list of blocks that define it
c.) insert Ø-functions
 global name n
 block b in which n is assigned
 block d in b’s dominance frontier
insert a Ø-function for n in d
add d to n’s list of defining blocks
Needs a little more detail
COMP 512, Fall 2003 *

13. SSA Construction Algorithm
Finding global names
Different between two forms of SSA
Minimal uses all names
Semi-pruned uses names that are live on entry to some block
Shrinks name space & number of Ø-functions
Pays for itself in compile-time speed
For each “global name”, need a list of blocks where it is defined
Drives Ø-function insertion
b defines x implies a Ø-function for x in every c  DF(b)
Pruned SSA adds a test to see if x is live at insertion point
Otherwise, cannot need a Ø-function
Occasionally, building pruned is faster than building semi-pruned.
Any algorithm that has non-linear behavior in the number of Ø-functions will have a size where pruned is the SSA flavor of choice.
COMP 512, Fall 2003

14. Excluding local names avoids Ø’s for y & z
a  Ø(a,a)
b  Ø(b,b)
c  Ø(c,c)
d  Ø(d,d)
y  a+b
z  c+d
i  i+1 B7
i > 100
i  ••• B0
b  •••
c  •••
d  ••• B2
i  Ø(i,i)
a  ••• B1 B3 B4 B5 B6
Example
Excluding local names avoids Ø’s for y & z
With all the Ø-functions
Lots of new ops
Renaming is next
Assume a, b, c, & d defined before B0

15. SSA Construction Algorithm (Less high-level sketch)
2. Rename variables in a pre-order walk over dominator tree
(use an array of stacks, one stack per global name)
Staring with the root block, b
a.) generate unique names for each Ø-function
and push them on the appropriate stacks
b.) rewrite each operation in the block
i. Rewrite uses of global names with the current version
(from the stack)
ii. Rewrite definition by inventing & pushing new name
c.) fill in Ø-function parameters of successor blocks
d.) recurse on b’s children in the dominator tree
e.) <on exit from block b > pop names generated in b from stacks
1 counter per name for subscripts
Reset the state
Need the end-of-block name for this path
COMP 512, Fall 2003

16. SSA Construction Algorithm (Less high-level sketch)
Adding all the details ...
Rename(b)
for each Ø-function in b, x  Ø(…)
rename x as NewName(x)
for each operation “x  y op z” in b
rewrite y as top(stack[y])
rewrite z as top(stack[z])
rewrite x as NewName(x)
for each successor of b in the CFG
rewrite appropriate Ø parameters
for each successor s of b in dom. tree
Rename(s)
pop(stack[x])
for each global name i
counter[i]  0
stack[i]  Ø
call Rename(n0)
NewName(n)
i  counter[n]
counter[n]  counter[n] + 1
push ni onto stack[n]
return ni
COMP 512, Fall 2003

17. Example Before processing B0 Assume a, b, c, & d defined before B0
a  Ø(a,a)
b  Ø(b,b)
c  Ø(c,c)
d  Ø(d,d)
y  a+b
z  c+d
i  i+1 B7
i > 100
i  ••• B0
b  •••
c  •••
d  ••• B2
i  Ø(i,i)
a  ••• B1 B3 B4 B5 B6
Example
Before processing B0
Assume a, b, c, & d defined before B0
i has not been defined a b c d i
Counters
Stacks 1 1 1 1 a0 b0 c0 d0 *

18. Example End of B0 a b c d i Counters Stacks 1 1 1 1 1 * a0 b0 c0 d0 i0
a  Ø(a0,a)
b  Ø(b0,b)
c  Ø(c0,c)
d  Ø(d0,d)
i  Ø(i0,i)
a  •••
c  ••• B1
End of B0
b  •••
c  •••
d  ••• B2
a  •••
d  ••• B3
d  ••• B4
c  ••• B5
d  Ø(d,d)
c  Ø(c,c)
b  ••• B6 a b c d i
Counters
Stacks 1 1 1 1 1 B7
a  Ø(a,a)
b  Ø(b,b)
c  Ø(c,c)
d  Ø(d,d)
y  a+b
z  c+d
i  i+1 a0 b0 c0 d0 i0
i > 100 *

19. Example End of B1 a b c d i Counters Stacks 3 2 3 2 2 * a0 b0 c0 d0 i0
a1  Ø(a0,a)
b1  Ø(b0,b)
c1  Ø(c0,c)
d1  Ø(d0,d)
i1  Ø(i0,i)
a2  •••
c2  ••• B1
End of B1
b  •••
c  •••
d  ••• B2
a  •••
d  ••• B3
d  ••• B4
c  ••• B5
d  Ø(d,d)
c  Ø(c,c)
b  ••• B6 a b c d i
Counters
Stacks 3 2 3 2 2 B7
a  Ø(a,a)
b  Ø(b,b)
c  Ø(c,c)
d  Ø(d,d)
y  a+b
z  c+d
i  i+1 a0 b0 c0 d0 i0 a1 b1 c1 d1 i1 a2 c2
i > 100 *

20. Example End of B2 a b c d i Counters Stacks 3 3 4 3 2 * a0 b0 c0 d0 i0
a1  Ø(a0,a)
b1  Ø(b0,b)
c1  Ø(c0,c)
d1  Ø(d0,d)
i1  Ø(i0,i)
a2  •••
c2  ••• B1
End of B2
b2  •••
c3  •••
d2  ••• B2
a  •••
d  ••• B3
d  ••• B4
c  ••• B5
d  Ø(d,d)
c  Ø(c,c)
b  ••• B6 a b c d i
Counters
Stacks 3 3 4 3 2 B7
a  Ø(a2,a)
b  Ø(b2,b)
c  Ø(c3,c)
d  Ø(d2,d)
y  a+b
z  c+d
i  i+1 a0 b0 c0 d0 i0 a1 b1 c1 d1 i1 a2 b2 c2 d2 c3
i > 100 *

21. Example Before starting B3 a b c d i Counters Stacks 3 3 4 3 2 * a0 b0
a1  Ø(a0,a)
b1  Ø(b0,b)
c1  Ø(c0,c)
d1  Ø(d0,d)
i1  Ø(i0,i)
a2  •••
c2  ••• B1
Before starting B3
b2  •••
c3  •••
d2  ••• B2
a  •••
d  ••• B3
d  ••• B4
c  ••• B5
d  Ø(d,d)
c  Ø(c,c)
b  ••• B6 a b c d i
Counters
Stacks 3 3 4 3 2 B7
a  Ø(a2,a)
b  Ø(b2,b)
c  Ø(c3,c)
d  Ø(d2,d)
y  a+b
z  c+d
i  i+1
i ≤ 100 a0 b0 c0 d0 i0 a1 b1 c1 d1 i1 a2 c2
i > 100 *

22. Example End of B3 a b c d i Counters Stacks 4 3 4 4 2 * a0 b0 c0 d0 i0
a1  Ø(a0,a)
b1  Ø(b0,b)
c1  Ø(c0,c)
d1  Ø(d0,d)
i1  Ø(i0,i)
a2  •••
c2  ••• B1
End of B3
b2  •••
c3  •••
d2  ••• B2
a3  •••
d3  ••• B3
d  ••• B4
c  ••• B5
d  Ø(d,d)
c  Ø(c,c)
b  ••• B6 a b c d i
Counters
Stacks 4 3 4 4 2 B7
a  Ø(a2,a)
b  Ø(b2,b)
c  Ø(c3,c)
d  Ø(d2,d)
y  a+b
z  c+d
i  i+1 a0 b0 c0 d0 i0 a1 b1 c1 d1 i1 a2 c2 d3 a3
i > 100 *

23. Example End of B4 a b c d i Counters Stacks 4 3 4 5 2 * a0 b0 c0 d0 i0
a1  Ø(a0,a)
b1  Ø(b0,b)
c1  Ø(c0,c)
d1  Ø(d0,d)
i1  Ø(i0,i)
a2  •••
c2  ••• B1
End of B4
b2  •••
c3  •••
d2  ••• B2
a3  •••
d3  ••• B3
d4  ••• B4
c  ••• B5
d  Ø(d4,d)
c  Ø(c2,c)
b  ••• B6 a b c d i
Counters
Stacks 4 3 4 5 2 B7
a  Ø(a2,a)
b  Ø(b2,b)
c  Ø(c3,c)
d  Ø(d2,d)
y  a+b
z  c+d
i  i+1 a0 b0 c0 d0 i0 a1 b1 c1 d1 i1 a2 c2 d3 a3 d4
i > 100 *

24. Example End of B5 a b c d i Counters Stacks 4 3 5 5 2 * a0 b0 c0 d0 i0
a1  Ø(a0,a)
b1  Ø(b0,b)
c1  Ø(c0,c)
d1  Ø(d0,d)
i1  Ø(i0,i)
a2  •••
c2  ••• B1
End of B5
b2  •••
c3  •••
d2  ••• B2
a3  •••
d3  ••• B3
d4  ••• B4
c4  ••• B5
d  Ø(d4,d3)
c  Ø(c2,c4)
b  ••• B6 a b c d i
Counters
Stacks 4 3 5 5 2 B7
a  Ø(a2,a)
b  Ø(b2,b)
c  Ø(c3,c)
d  Ø(d2,d)
y  a+b
z  c+d
i  i+1 a0 b0 c0 d0 i0 a1 b1 c1 d1 i1 a2 c2 d3 a3 c4
i > 100 *

25. Example End of B6 a b c d i Counters Stacks 4 4 6 6 2 * a0 b0 c0 d0 i0
a1  Ø(a0,a)
b1  Ø(b0,b)
c1  Ø(c0,c)
d1  Ø(d0,d)
i1  Ø(i0,i)
a2  •••
c2  ••• B1
End of B6
b2  •••
c3  •••
d2  ••• B2
a3  •••
d3  ••• B3
d4  ••• B4
c4  ••• B5
d5  Ø(d4,d3)
c5  Ø(c2,c4)
b3  ••• B6 a b c d i
Counters
Stacks 4 4 6 6 2 B7
a  Ø(a2,a3)
b  Ø(b2,b3)
c  Ø(c3,c5)
d  Ø(d2,d5)
y  a+b
z  c+d
i  i+1 a0 b0 c0 d0 i0 a1 b1 c1 d1 i1 a2 b3 c2 d3 a3 c5 d5
i > 100 *

26. Example Before B7 a b c d i Counters Stacks 4 4 6 6 2 * a0 b0 c0 d0 i0
a1  Ø(a0,a)
b1  Ø(b0,b)
c1  Ø(c0,c)
d1  Ø(d0,d)
i1  Ø(i0,i)
a2  •••
c2  ••• B1
Before B7
b2  •••
c3  •••
d2  ••• B2
a3  •••
d3  ••• B3
d4  ••• B4
c4  ••• B5
d5  Ø(d4,d3)
c5  Ø(c2,c4)
b3  ••• B6 a b c d i
Counters
Stacks 4 4 6 6 2 B7
a  Ø(a2,a3)
b  Ø(b2,b3)
c  Ø(c3,c5)
d  Ø(d2,d5)
y  a+b
z  c+d
i  i+1 a0 b0 c0 d0 i0 a1 b1 c1 d1 i1 a2 c2
i > 100 *

27. Example End of B7 a b c d i Counters Stacks 5 5 7 7 3 * a0 b0 c0 d0 i0
a1  Ø(a0,a4)
b1  Ø(b0,b4)
c1  Ø(c0,c6)
d1  Ø(d0,d6)
i1  Ø(i0,i2)
a2  •••
c2  ••• B1
End of B7
b2  •••
c3  •••
d2  ••• B2
a3  •••
d3  ••• B3
d4  ••• B4
c4  ••• B5
d5  Ø(d4,d3)
c5  Ø(c2,c4)
b3  ••• B6 a b c d i
Counters
Stacks 5 5 7 7 3 B7
a4  Ø(a2,a3)
b4  Ø(b2,b3)
c6  Ø(c3,c5)
d6  Ø(d2,d5)
y  a4+b4
z  c6+d6
i2  i1+1 a0 b0 c0 d0 i0 a1 b1 c1 d1 i1 a2 b4 c2 d6 i2 a4 c6
i > 100 *

28. Example After renaming Semi-pruned SSA form We’re done … CountersB0
i0  •••
i > 100
Example
a1  Ø(a0,a4)
b1  Ø(b0,b4)
c1  Ø(c0,c6)
d1  Ø(d0,d6)
i1  Ø(i0,i2)
a2  •••
c2  ••• B1
After renaming
Semi-pruned SSA form
We’re done …
b2  •••
c3  •••
d2  ••• B2
a3  •••
d3  ••• B3
d4  ••• B4
c4  ••• B5
d5  Ø(d4,d3)
c5  Ø(c2,c4)
b3  ••• B6
Counters
Stacks B7
a4  Ø(a2,a3)
b4  Ø(b2,b3)
c6  Ø(c3,c5)
d6  Ø(d2,d5)
y  a4+b4
z  c6+d6
i2  i1+1
Semi-pruned  only names live in 2 or more blocks are “global names”.
i > 100

29. SSA Construction Algorithm (Pruned SSA)
What’s this “pruned SSA” stuff?
Minimal SSA still contains extraneous Ø-functions
Inserts some Ø-functions where they are dead
Would like to avoid inserting them
Two ideas
Semi-pruned SSA: discard names used in only one block
Significant reduction in total number of Ø-functions
Needs only local Live information (cheap to compute)
Pruned SSA: only insert Ø-functions where their value is live
Inserts even fewer Ø-functions, but costs more to do
Requires global Live variable analysis (more expensive)
In practice, both are simple modifications to step 1.
COMP 512, Fall 2003

30. SSA Construction Algorithm
We can improve the stack management
Push at most one name per stack per block (save push & pop)
Thread names together by block
To pop names for block b, use b’s thread
This is another good use for a scoped hash table
Significant reductions in pops and pushes
Makes a minor difference in SSA construction time
Scoped table is a clean, clear way to handle the problem
COMP 512, Fall 2003

31. SSA Deconstruction At some point, we need executable code
Few machines implement Ø operations
Need to fix up the flow of values
Basic idea
Insert copies Ø-function pred’s
Simple algorithm
Works in most cases
Adds lots of copies
Most of them coalesce away
X17 Ø(x10,x11)
...  x17
...
...  x17
X17  x10
X17  x11
COMP 512, Fall 2003 *