[llvm-dev] Where's the optimiser gone? (part 5.c): missed tail calls, and more...

[Stefan Kanthak via llvm-dev]

"Craig Topper" <craig.topper at gmail.com> wrote:

> For the multiply case, your improved code requires duplicating a load.

... while saving another load and a store, and it doesn't clobber ESI.

> Sure it's safe in this case because there are no stores and the memory isn't
> volatile.

Right. Especially the latter can safely be assumed true for function
arguments on the stack.

> But the register allocator would have to analyze the code to
> prove that it's safe to duplicate.

This sentence implies for me that it might be a design goal to
never load a function argument or variable twice.
If yes, that's OK; if not, the register allocator should prove.

regards
Stefan

> On Sat, Dec 1, 2018 at 9:38 AM Stefan Kanthak via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
> 
>> Compile the following functions with "-O3 -target i386-win32"
>> (see <https://godbolt.org/z/exmjWY>):
>>
>> __int64 __fastcall div(__int64 foo, __int64 bar)
>> {
>>     return foo / bar;
>> }
>>
>>
>> On the left the generated code; on the right the expected,
>> properly optimised code:
>>
>>     push  dword ptr [esp + 16]        |
>>     push  dword ptr [esp + 16]        |
>>     push  dword ptr [esp + 16]        |
>>     push  dword ptr [esp + 16]        |
>>     call  __alldiv                    |     jmp  __alldiv
>>     ret   16                          |
>>
>>
>> __int64 __fastcall mod(__int64 foo, __int64 bar)
>> {
>>     return foo % bar;
>> }
>>
>>     push  dword ptr [esp + 16]        |
>>     push  dword ptr [esp + 16]        |
>>     push  dword ptr [esp + 16]        |
>>     push  dword ptr [esp + 16]        |
>>     call  __allrem                    |     jmp  __allrem
>>     ret   16                          |
>>
>>
>> __int64 __fastcall mul(__int64 foo, __int64 bar)
>> {
>>     return foo * bar;
>> }
>>
>>     push  esi                         |     mov   ecx, dword ptr [esp + 16]
>>     mov   ecx, dword ptr [esp + 16]   |     mov   edx, dword ptr [esp + 12]
>>     mov   esi, dword ptr [esp + 8]    |     imul  edx, dword ptr [esp + 8]
>>     mov   eax, ecx                    |     mov   eax, dword ptr [esp + 4]
>>     imul  ecx, dword ptr [esp + 12]   |     imul  ecx, eax
>>     mul   esi                         |     add   ecx, edx
>>     imul  esi, dword ptr [esp + 20]   |     mul   dword ptr [esp + 12]
>>     add   edx, ecx                    |     add   edx, ecx
>>     add   edx, esi                    |     ret   16
>>     pop   esi                         |
>>     ret   16                          |
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