On Sun, Aug 20, 2017 at 03:54:47PM +0100, Dibyendu Majumdar wrote:
> Hi,
>
> I am trying to get to grips with the Sparse Linear format and the use
> of pseudos.
>
> 1) A Sparse document says that pseudos are akin to SSA variables. Is
> that a true statement - i.e. do pseudos follow the discipline that
> only one assignment to a pseudo is allowed. As pseudos have also
> different sub-types - does this statement apply to all pseudo types or
> only some sub types?
>
> 2) In a recent conversation it was stated that the baseline Linear
> output from Sparse is already in SSA form. That implies that all
> pseudos used are in SSA form already. However we know that in the
> baseline line IR phi nodes may not be present. In LLVM the initial IR
> lacks phi nodes too - instead local stack memory and load/store
> sequences are used. However LLVM IR is still SSA at this stage. When
> it is said that the baseline Linear IR is already SSA is it in this
> sense?
One way to answer this with few words is to look at what's happening
to a variable assigned in different paths in a simple example:
int f(void);
int foo(int p)
{
int a;
if (p) {
a = 0;
a++;
} else {
a = f();
}
return a;
}
Just after linearization:
foo:
.L0:
<entry-point>
store.32 %arg1 -> 0[p]
load.32 %r1 <- 0[p]
cbr %r1, .L1, .L2
.L1:
store.32 $0 -> 0[a]
load.32 %r2 <- 0[a]
add.32 %r3 <- %r2, $1
store.32 %r3 -> 0[a]
br .L3
.L2:
call.32 %r4 <- f
store.32 %r4 -> 0[a]
br .L3
.L3:
load.32 %r5 <- 0[a]
ret.32 %r5
After conversion to SSA form:
foo:
.L0:
<entry-point>
cbr %arg1, .L1, .L2
.L1:
add.32 %r3(a) <- $0, $1
phisrc.32 %phi2(a) <- %r3(a)
br .L3
.L2:
call.32 %r4 <- f
phisrc.32 %phi3(a) <- %r4
br .L3
.L3:
phi.32 %r5 <- %phi2(a), %phi3(a)
ret.32 %r5
After simplification:
foo:
.L0:
<entry-point>
cbr %arg1, .L1, .L2
.L1:
phisrc.32 %phi2(a) <- $1
br .L3
.L2:
call.32 %r4 <- f
phisrc.32 %phi3(a) <- %r4
br .L3
.L3:
phi.32 %r5 <- %phi2(a), %phi3(a)
ret.32 %r5
After going out of SSA (currently not used):
foo:
.L0
<entry-point>
cbr %arg1, .L1, .L2
.L1
copy.32 %r7 <- $1
br .L3
.L2
call.32 %r7 <- f
br .L3
.L3
ret.32 %r7
-- Luc
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