On 7 September 2017 at 21:25, Luc Van Oostenryck
<luc.vanoostenryck@xxxxxxxxx> wrote:
> 2017-09-07 18:04 GMT+02:00 Dibyendu Majumdar <mobile@xxxxxxxxxxxxxxx>:
>> The following code snippet:
>>
>> static int test_do(void) {
>> int a = 0;
>> int count = 27;
>> switch (count % 8) {
>> case 0: do { a++;
>> case 7: a++;
>> case 6: a++;
>> case 5: a++;
>> case 4: a++;
>> case 3: a++;
>> case 2: a++;
>> case 1: a++;
>> } while ((count -= 8) > 0);
>> }
>> return a;
>> }
>>
>> Results in following linear output:
>>
>> test_do:
>> .L0:
>> <entry-point>
>> phisrc.32 %phi3(a) <- $0
>> phisrc.32 %phi4(a) <- $0
>> phisrc.32 %phi6(a) <- $0
>> phisrc.32 %phi8(a) <- $0
>> phisrc.32 %phi10(a) <- $0
>> phisrc.32 %phi12(a) <- $0
>> phisrc.32 %phi14(a) <- $0
>> phisrc.32 %phi16(a) <- $0
>> phisrc.32 %phi18(a) <- $0
>> phisrc.32 %phi20(count) <- $27
>> br .L7
>>
>> .L10:
>> phi.32 %r3 <- %phi18(a), %phi19(a)
>> add.32 %r4 <- %r3, $1
>> phisrc.32 %phi17(a) <- %r4
>> phi.32 %r5 <- %phi16(a), %phi17(a)
>> ...
>>
>> This is translated to following LLVM IR:
>>
>> It seems to me that 'a' ends up having 9 different slots on the stack,
>> but it should really be merged into one slot, i.e. we should detect
>> that the phis all related to the same symbol, and create only one
>> alloca?
>
>
> Yes and no. It's just one of the problem with the SSA conversion
> as done in rc5.
> If you try the same code with newssa, you have something much
> saner, no?
>
Seems worse actually:
Here is a part of the linearized output pre simplifications. Note that
I haven't applied the fix you sent yesterday.
test_do:
.L0:
<entry-point>
mods.32 %r1 <- $27, $8
phisrc.32 %phi1(a) <- $0
phisrc.32 %phi3(a) <- $0
phisrc.32 %phi5(a) <- $0
phisrc.32 %phi7(a) <- $0
phisrc.32 %phi9(a) <- $0
phisrc.32 %phi11(a) <- $0
phisrc.32 %phi13(a) <- $0
phisrc.32 %phi15(count) <- $27
phisrc.32 %phi16(count) <- $27
phisrc.32 %phi17(count) <- $27
phisrc.32 %phi18(count) <- $27
phisrc.32 %phi19(count) <- $27
phisrc.32 %phi20(count) <- $27
phisrc.32 %phi21(count) <- $27
phisrc.32 %phi34(a) <- $0
switch.32 %r1, 0 -> .L2, 1 -> .L9, 2 -> .L8, 3 -> .L7, 4 -> .L6, 5
-> .L5, 6 -> .L4, 7 -> .L3, default -> .L1
.L2:
phisrc.32 %phi29(a) <- $0
phisrc.32 %phi31(count) <- $27
br .L10
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