[llvm-dev] Where's the optimiser gone? (part 5.b): missed tail calls, and more... (original) (raw)
Craig Topper via llvm-dev llvm-dev at lists.llvm.org
Sat Dec 1 11:07:33 PST 2018
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Clang's -target option is supposed to take a cpu type and an operating system. So "-target i386" is giving it no operatiing system. This is preventing frame pointer elimination which is why ebp is being updated. If you pass "-target i386-linux" you get sightly better code.
The division/remainder operations are turned into library calls as part of instruction selection. This code is somewhat independent of how other calls are handled. We probably don't support tail calls in it. Is it really realistic that a user would have a non-inlined function that contains just a division? Why should we optimize for that case?
~Craig
On Sat, Dec 1, 2018 at 9:37 AM Stefan Kanthak via llvm-dev < llvm-dev at lists.llvm.org> wrote:
Compile the following functions with "-O3 -target i386" (see <https://godbolt.org/z/VmKlXL>):
long long div(long long foo, long long bar) { return foo / bar; } On the left the generated code; on the right the expected, properly optimised code: div: # @div push ebp | mov ebp, esp | push dword ptr [ebp + 20] | push dword ptr [ebp + 16] | push dword ptr [ebp + 12] | push dword ptr [ebp + 8] | _call _divdi3 | jmp divdi3 add esp, 16 | pop ebp | ret |
long long mod(long long foo, long long bar) { return foo % bar; } mod: # @mod push ebp | mov ebp, esp | push dword ptr [ebp + 20] | push dword ptr [ebp + 16] | push dword ptr [ebp + 12] | push dword ptr [ebp + 8] | _call _moddi3 | jmp moddi3 add esp, 16 | pop ebp | ret | long long mul(long long foo, long long bar) { return foo * bar; } mul: # @mul push ebp mov ebp, esp push esi mov ecx, dword ptr [ebp + 16] mov esi, dword ptr [ebp + 8] mov eax, ecx imul ecx, dword ptr [ebp + 12] mul esi imul esi, dword ptr [ebp + 20] add edx, ecx add edx, esi pop esi pop ebp ret
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