IR PGO not working on Windows

[JohanEngelen]

IR PGO is disabled on Windows right now, see #2474

@Hardcode84 found a testcase, demonstrating (most likely) a bug in LLVM:

// File: main.d
int foo()
{
  return 42;
}

ldc2 main -output-ll output:

; ModuleID = 'main.d'
source_filename = "main.d"
target datalayout = "e-m:w-i64:64-f80:128-n8:16:32:64-S128"
target triple = "x86_64-pc-windows-msvc"

%0 = type { i32, i32, [5 x i8] }
%object.ModuleInfo = type { i32, i32 }

$_D4main3fooFZi = comdat any

$_D4main12__ModuleInfoZ = comdat any

@_D4main12__ModuleInfoZ = global %0 { i32 -2147483644, i32 0, [5 x i8] c"main\00" }, comdat ; [#uses = 1]
@_D4main11__moduleRefZ = linkonce_odr global %object.ModuleInfo* bitcast (%0* @_D4main12__ModuleInfoZ to %object.ModuleInfo*), section ".minfo" ; [#uses = 1]
@llvm.used = appending global [1 x i8*] [i8* bitcast (%object.ModuleInfo** @_D4main11__moduleRefZ to i8*)], section "llvm.metadata" ; [#uses = 0]

; [#uses = 0]
; Function Attrs: uwtable
define i32 @_D4main3fooFZi() #0 comdat {
  ret i32 42
}

attributes #0 = { uwtable "less-precise-fpmad"="false" "no-frame-pointer-elim"="true" "no-infs-fp-math"="false" "no-nans-fp-math"="false" "target-cpu"="x86-64" "target-features"="+cx16" "unsafe-fp-math"="false" }

!llvm.linker.options = !{}
!llvm.ident = !{!0}

!0 = !{!"ldc version 1.7.0git-43c02ce-dirty"}

opt -pgo-instr-gen -instrprof -verify main.ll -S output:

comdat global value has private linkage
[1 x i64]* @__profc__D4main3fooFZi
LLVM ERROR: Broken module found, compilation aborted!

Error goes away if comdat is removed from foo.

[JohanEngelen]

reported here: https://bugs.llvm.org/show_bug.cgi?id=36120

[kinke]

For AST-based PGO, we override the linkage specifically here.

[kinke]

It might be easily worked around in our LLVM fork, by modding these 3 lines. I'll check it out.

[kinke]

Apparently so, PASS: LDC :: PGO/irbased_indirect_calls.d. Not sure if I'll do a third LLVM 6.0.1 release or delay it until LLVM 7.

[jondegenhardt]

An issue reference, in the off-chance this would help the IR-PGO+LTO case: #2582

[kinke]

Resolved by #2795.