[DTLTO] Forward loop interchange to DTLTO remote compilations (#208591)

Recent upstream LLVM changes enabled `LoopInterchangePass` by default
through `PipelineTuningOptions`. TLTO observes that state directly from
the LTO config, but DTLTO was not forwarding the equivalent Clang option
to the remote compiler. As a result, TLTO could run loop interchange
pass while DTLTO did not, producing codegen differences for
configurations that exposed profitable/legally interchangeable loops.
Note that this was a pre-existing issue, the change in default has
simply exposed it.

Forward `C.PTO.LoopInterchange` as `-floop-interchange` when
constructing the common DTLTO remote compiler options. Extend the
testing to check that the SN-DBS remote command line includes the
forwarded option.

The code for forwarding such options is only intended as a temporary
measure. A more comprehensive solution is in discussion here:
https://discourse.llvm.org/t/synchronizing-lto-code-generation-configuration-between-clang-and-lld-dtlto

cross-project-tests/dtlto/remote-options.test

@@ -47,6 +47,15 @@ NO-SECTIONS-NOT: "-faddrsig"
 NO-SECTIONS-NOT: "-ffunction-sections"
 NO-SECTIONS-NOT: "-fdata-sections"
 
+## Loop interchange.
+RUN: %{lto2-elf} -enable-loopinterchange=true 2>&1 | \
+RUN:   FileCheck %s --check-prefixes=ARGS,LOOP-INTERCHANGE,ERR
+LOOP-INTERCHANGE: "-floop-interchange"
+
+RUN: %{lto2-elf} -enable-loopinterchange=false 2>&1 | \
+RUN:   FileCheck %s --check-prefixes=ARGS,NO-LOOP-INTERCHANGE,ERR
+NO-LOOP-INTERCHANGE-NOT: "-floop-interchange"
+
 ## PIC is forwarded as -fpic for ELF targets but not for COFF targets because
 ## Clang does not accept -fpic for every target.
 RUN: %{lto2-elf} -relocation-model=pic 2>&1 | \

llvm/lib/DTLTO/DTLTO.cpp

@@ -227,6 +227,8 @@ void lto::DTLTO::buildCommonRemoteCompilerOptions() {
     Ops.push_back("-ffunction-sections");
   if (C.Options.DataSections)
     Ops.push_back("-fdata-sections");
+  if (C.PTO.LoopInterchange)
+    Ops.push_back("-floop-interchange");
 
   if (C.RelocModel == Reloc::PIC_)
     // Clang doesn't have -fpic for all triples.

llvm/test/ThinLTO/X86/dtlto/json.ll

@@ -37,6 +37,7 @@ LLVM-NEXT:  "clang"
 CHECK-NEXT: "-c"
 CHECK-NEXT: "--target=x86_64-unknown-linux-gnu"
 CHECK-NEXT: "-O2"
+CHECK-NEXT: "-floop-interchange"
 CHECK-NEXT: "-fpic"
 CHECK-NEXT: "-Wno-unused-command-line-argument"
 CHECK-NEXT: "--rota1=10"