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

Stefan Kanthak via llvm-dev llvm-dev at lists.llvm.org
Sat Dec 1 12:15:09 PST 2018

"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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