[Bug c++/90216] New: Stack Pointer decrementing even when not loading extra data to stack.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90216 Bug ID: 90216 Summary: Stack Pointer decrementing even when not loading extra data to stack. Product: gcc Version: 8.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: stevexiong98 at hotmail dot com Target Milestone: --- Hi, Using ARM GCC 8.2, when I instantiate an object the corresponding Assembly instructions involve a decrementing, then incrementing of the stack pointer. However, no values are being transferred between the registers and the empty stack space. Please check out this link for details, lines 5 and 7 in the Assembly panel show how the stack pointer is decremented/incremented unnecessarily. https://godbolt.org/z/h-H7Ox In the C++ panel when you comment out line 53 and uncomment the line below, the Assembly instructions involving the stack pointer disappear. The same is true if you uncomment just line 55. Can you please explain why the stack pointer inc/dec operations are not optimized out in the first line of code (line 53)? Can you please try to release a patch where this unnecessary stack pointer inc/dec is no longer an issue? Thanks
[Bug c++/90217] New: Greater optimization of C++ Code
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90217 Bug ID: 90217 Summary: Greater optimization of C++ Code Product: gcc Version: 8.2.0 Status: UNCONFIRMED Severity: normal Priority: P3 Component: c++ Assignee: unassigned at gcc dot gnu.org Reporter: stevexiong98 at hotmail dot com Target Milestone: --- Hi, This is not so much a bug, but more of an enhancement. There are 2 pieces of code I have listed below which should translate to identical assembly instructions at high levels of compiler optimization (level 3) but currently do not. https://godbolt.org/z/Zn7FMK https://godbolt.org/z/wB8eZd Using ARM GCC 8.2, the code in the second link involves the stack pointer and extra load/store operations to the newly-created stack space. There are more assembly instructions in link 2's code than in link 1's code. However, in Godbolt if you switch the compiler to Clang, at optimization 3 both pieces of code manage to compile down to the same minimal Assembly instructions. Switching the compiler to x86-64 GCC (trunk), the code in the second link also has a few extra operations compared the first link's code. Is it possible to set ARM GCC and x86-64 GCC to a particular optimization setting that allows both links' code to have matching assembly instructions? If not, is it possible that in a future release, both compilers could apply enough optimizations so that the assembly in link 1 matches link 2? Thanks
[Bug middle-end/90216] Stack Pointer decrementing even when not loading extra data to stack.
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90216 --- Comment #2 from stevexiong98 at hotmail dot com --- (In reply to Andrew Pinski from comment #1) > Confirmed, part of the problem is union is forced to memory early on but > then optimized out but the object still exist in memory even though it is > dead. > I am working on an optimization which improves this by the lowering of > bit-fields. But it won't go in until post GCC 9 (released next year). Thanks so much for the update. I am looking forward to the next compiler release!
