[llvm-bugs] [Bug 51827] New: Huge codegen performance regression for Windows targets since LLVM 12, incl. latest 13-rc2

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=51827

            Bug ID: 51827
           Summary: Huge codegen performance regression for Windows
                    targets since LLVM 12, incl. latest 13-rc2
           Product: new-bugs
           Version: trunk
          Hardware: PC
                OS: Windows NT
            Status: NEW
          Severity: release blocker
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: kinke at gmx.net
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

Created attachment 25252
  --> https://bugs.llvm.org/attachment.cgi?id=25252&action=edit
xz-compressed bitcode input file

I'm seeing a codegen slowdown by about 2 orders of magnitude for the attached
.bc file (generated by LDC, the LLVM D compiler) starting with LLVM 12.0.0.
Seems to be specific to Windows/MSVC targets, and timings are back to normal
when enabling the optimizer.

Using clang from the official LLVM Windows builds from GitHub and compiling via
`clang -c template9.bc`:
* v11.0.1: ~2.4 seconds
* v12.0.1: ~246 seconds
* v13.0.0-rc2: ~227 seconds

Enabling the optimizer by adding `-O3` hints at a further little regression in
v13:
* v11.0.1: ~3.3 seconds
* v12.0.1: ~3.3 seconds
* v13.0.0-rc2: ~5.0 seconds

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