Hi Chris,
On 16 August 2017 at 05:58, Christopher Li <sparse@xxxxxxxxxxx> wrote:
> On Wed, Aug 16, 2017 at 12:23 AM, Christopher Li <sparse@xxxxxxxxxxx> wrote:
>> If sparse only place the phi node on where the loads are, which a lot of
>> case happen to be the DF (dominance frontier). In the case that DF
>> does not have loads, that is where current sparse do wrong, sparse
>> does not place a phi node there. Which violate the SSA property.
>
> I give an example for the above situation I am talking about:
>
> int a, b, c, d;
>
> void foo(void)
> {
> int e, f;
> if (a)
> e = b;
> else
> e = c;
> if (b)
> c = a+b;
> d = e;
> }
>
> "if (b)" is where the dominance of "e = b" and "e =c" joined.
> I don't do any loading of "e" there.
> To insert some blocks before using "e", I use the "if (b)".
>
> Now let's look at the test-linearize output:
I think the linear output you obtained was post simplification. We
need to look at the output prior to any simplifications. I believe it
looks like this:
foo:
.L0:
<entry-point>
load.32 %r1 <- 0[a]
cbr %r1, .L1, .L2
.L1:
load.32 %r2 <- 0[b]
store.32 %r2 -> 0[e]
br .L3
.L2:
load.32 %r3 <- 0[c]
store.32 %r3 -> 0[e]
br .L3
.L3:
load.32 %r4 <- 0[b]
cbr %r4, .L4, .L5
.L4:
load.32 %r5 <- 0[a]
load.32 %r6 <- 0[b]
add.32 %r7 <- %r5, %r6
store.32 %r7 -> 0[c]
br .L5
.L5:
load.32 %r8 <- 0[e]
store.32 %r8 -> 0[d]
br .L6
.L6:
ret
And here is the output from the new SSA construction:
foo:
.L0:
<entry-point>
load.32 %r1 <- 0[a]
cbr %r1, .L1, .L2
.L1:
load.32 %r2 <- 0[b]
phisrc.32 %phi1(e) <- %r2
br .L3
.L2:
load.32 %r3 <- 0[c]
phisrc.32 %phi2(e) <- %r3
br .L3
.L3:
phi.32 %r9(e) <- %phi1(e), %phi2(e)
load.32 %r4 <- 0[b]
cbr %r4, .L4, .L5
.L4:
load.32 %r5 <- 0[a]
load.32 %r6 <- 0[b]
add.32 %r7 <- %r5, %r6
store.32 %r7 -> 0[c]
br .L5
.L5:
store.32 %r9(e) -> 0[d]
br .L6
.L6:
ret
Finally here is what clang does after some phases (initially clang
starts with local vars).
*** IR Dump After SROA ***
; Function Attrs: nounwind
define void @foo() #0 {
entry:
%0 = load i32, i32* @a, align 4, !tbaa !1
%tobool = icmp ne i32 %0, 0
br i1 %tobool, label %if.then, label %if.else
if.then: ; preds = %entry
%1 = load i32, i32* @b, align 4, !tbaa !1
br label %if.end
if.else: ; preds = %entry
%2 = load i32, i32* @c, align 4, !tbaa !1
br label %if.end
if.end: ; preds = %if.else, %if.then
%e.0 = phi i32 [ %1, %if.then ], [ %2, %if.else ]
%3 = load i32, i32* @b, align 4, !tbaa !1
%tobool1 = icmp ne i32 %3, 0
br i1 %tobool1, label %if.then2, label %if.end3
if.then2: ; preds = %if.end
%4 = load i32, i32* @a, align 4, !tbaa !1
%5 = load i32, i32* @b, align 4, !tbaa !1
%add = add nsw i32 %4, %5
store i32 %add, i32* @c, align 4, !tbaa !1
br label %if.end3
if.end3: ; preds = %if.then2, %if.end
store i32 %e.0, i32* @d, align 4, !tbaa !1
ret void
}
Regards
Dibyendu
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