Fix GCC 13 Release build test failures (microsoft#7364) (microsoft#8176)

GCC 13 (e.g. 13.3.0 shipped with Ubuntu 24.04) has a known
miscompilation bug in -funswitch-loops
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109934) that causes
widespread test failures when building DXC at -O3.

Disable -funswitch-loops for GCC 13, and update the CI pipeline from
Ubuntu-22.04 to Ubuntu-24.04 (which ships GCC 13 as default), removing
the workaround from microsoft#7365.

Fixes microsoft#7364

---------

Co-authored-by: Copilot <[email protected]>

Author: damyanp

azure-pipelines.yml

@@ -78,7 +78,7 @@ stages:
 
     variables:
       macOS: macOS-latest
-      linux: Ubuntu-22.04 # FIXME: #7364, DXC does not build correctly with GCC 13+
+      linux: Ubuntu-24.04
 
     strategy:
       matrix:
@@ -99,13 +99,13 @@ stages:
         Linux_Gcc_Release:
           image: ${{ variables.linux }}
           configuration: Release
-          CC: gcc-9
-          CXX: g++-9
+          CC: gcc
+          CXX: g++
         Linux_Gcc_Debug:
           image: ${{ variables.linux }}
           configuration: Debug
-          CC: gcc-9
-          CXX: g++-9
+          CC: gcc
+          CXX: g++
           CMAKE_OPTS: -DLLVM_USE_SPLIT_DWARF=On
         MacOS_Clang_Release:
           image: ${{ variables.macOS }}
@@ -144,11 +144,6 @@ stages:
       displayName: 'Installing dependencies'
       condition: eq(variables['image'], variables['linux'])
 
-    - bash: |
-        sudo apt-get install g++-9 -y
-      displayName: 'Installing g++-9'
-      condition: and(eq(variables['CXX'], 'g++-9'), eq(variables['image'], variables['linux']))
-
     - bash: |
         brew update
         brew install ninja

cmake/modules/HandleLLVMOptions.cmake

@@ -16,6 +16,14 @@ if(NOT LLVM_FORCE_USE_OLD_TOOLCHAIN)
     if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 4.7)
       message(FATAL_ERROR "Host GCC version must be at least 4.7!")
     endif()
+    # GCC 13 has a known miscompilation bug in -funswitch-loops
+    # (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109934) that causes
+    # widespread test failures at -O3. Disable -funswitch-loops for GCC 13.
+    if(CMAKE_CXX_COMPILER_VERSION VERSION_GREATER_EQUAL 13.0 AND
+       CMAKE_CXX_COMPILER_VERSION VERSION_LESS 14.0)
+      add_compile_options(-fno-unswitch-loops)
+      message(STATUS "Disabling -funswitch-loops due to GCC 13 miscompilation bug (PR109934)")
+    endif()
   elseif(CMAKE_CXX_COMPILER_ID STREQUAL "Clang")
     if(CMAKE_CXX_COMPILER_VERSION VERSION_LESS 3.1)
       message(FATAL_ERROR "Host Clang version must be at least 3.1!")