--- a/config/config.mk
+++ b/config/config.mk
@@ -181,17 +181,24 @@ ifneq (,$(MOZ_DEBUG)$(MOZ_DEBUG_SYMBOLS)
     endif
   else
     _DEBUG_ASFLAGS += $(MOZ_DEBUG_FLAGS)
   endif
   _DEBUG_CFLAGS += $(MOZ_DEBUG_FLAGS)
   _DEBUG_LDFLAGS += $(MOZ_DEBUG_LDFLAGS)
 endif
 
+ifeq ($(YASM),$(AS))
+# yasm doesn't like the GNU as flags we may already have in ASFLAGS, so reset.
+ASFLAGS := $(_DEBUG_ASFLAGS)
+# yasm doesn't like -c
+AS_DASH_C_FLAG=
+else
 ASFLAGS += $(_DEBUG_ASFLAGS)
+endif
 OS_CFLAGS += $(_DEBUG_CFLAGS)
 OS_CXXFLAGS += $(_DEBUG_CFLAGS)
 OS_LDFLAGS += $(_DEBUG_LDFLAGS)
 
 # XXX: What does this? Bug 482434 filed for better explanation.
 ifeq ($(OS_ARCH)_$(GNU_CC),WINNT_)
 ifdef MOZ_DEBUG
 ifneq (,$(MOZ_BROWSE_INFO)$(MOZ_BSCFILE))
@@ -423,17 +430,17 @@ OS_COMPILE_CMFLAGS += -fobjc-abi-version
 OS_COMPILE_CMMFLAGS += -fobjc-abi-version=2 -fobjc-legacy-dispatch
 endif
 endif
 
 COMPILE_CFLAGS	= $(VISIBILITY_FLAGS) $(DEFINES) $(INCLUDES) $(OS_INCLUDES) $(DSO_CFLAGS) $(DSO_PIC_CFLAGS) $(RTL_FLAGS) $(OS_CPPFLAGS) $(OS_COMPILE_CFLAGS) $(CFLAGS) $(MOZBUILD_CFLAGS) $(EXTRA_COMPILE_FLAGS)
 COMPILE_CXXFLAGS = $(if $(DISABLE_STL_WRAPPING),,$(STL_FLAGS)) $(VISIBILITY_FLAGS) $(DEFINES) $(INCLUDES) $(OS_INCLUDES) $(DSO_CFLAGS) $(DSO_PIC_CFLAGS) $(RTL_FLAGS) $(OS_CPPFLAGS) $(OS_COMPILE_CXXFLAGS) $(CXXFLAGS) $(MOZBUILD_CXXFLAGS) $(EXTRA_COMPILE_FLAGS)
 COMPILE_CMFLAGS = $(OS_COMPILE_CMFLAGS) $(MOZBUILD_CMFLAGS) $(EXTRA_COMPILE_FLAGS)
 COMPILE_CMMFLAGS = $(OS_COMPILE_CMMFLAGS) $(MOZBUILD_CMMFLAGS) $(EXTRA_COMPILE_FLAGS)
-ASFLAGS += $(EXTRA_ASSEMBLER_FLAGS)
+ASFLAGS += $(MOZBUILD_ASFLAGS) $(EXTRA_ASSEMBLER_FLAGS)
 
 ifndef CROSS_COMPILE
 HOST_CFLAGS += $(RTL_FLAGS)
 endif
 
 #
 # Name of the binary code directories
 #

--- a/configure.in
+++ b/configure.in
@@ -3861,17 +3861,16 @@ MOZ_SCTP=
 MOZ_ANDROID_OMX=
 MOZ_MEDIA_NAVIGATOR=
 MOZ_OMX_PLUGIN=
 MOZ_VPX=
 MOZ_VPX_ERROR_CONCEALMENT=
 MOZ_WEBSPEECH=1
 VPX_AS=
 VPX_ASFLAGS=
-VPX_AS_DASH_C_FLAG=
 VPX_AS_CONVERSION=
 VPX_ASM_SUFFIX=
 VPX_X86_ASM=
 VPX_ARM_ASM=
 LIBJPEG_TURBO_AS=
 LIBJPEG_TURBO_ASFLAGS=
 LIBJPEG_TURBO_X86_ASM=
 LIBJPEG_TURBO_X64_ASM=
@@ -5479,17 +5478,16 @@ if test -n "$MOZ_VPX" -a -z "$MOZ_NATIVE
       fi # COMPILE_ENVIRONMENT and others
     ;;
     *:arm*)
       if test -n "$GNU_AS" ; then
         VPX_AS=$AS
         dnl These flags are a lie; they're just used to enable the requisite
         dnl opcodes; actual arch detection is done at runtime.
         VPX_ASFLAGS="-march=armv7-a -mfpu=neon"
-        VPX_DASH_C_FLAG="-c"
         VPX_AS_CONVERSION='$(PERL) $(topsrcdir)/media/libvpx/build/make/ads2gas.pl'
         VPX_ASM_SUFFIX="$ASM_SUFFIX"
         VPX_ARM_ASM=1
       fi
     ;;
     *:x86)
       if $CC -E -dM -</dev/null | grep -q __ELF__; then
         VPX_ASFLAGS="-f elf32 -rnasm -pnasm -DPIC"
@@ -8859,27 +8857,26 @@ AC_SUBST(MOZ_FMP4)
 AC_SUBST(MOZ_EME)
 AC_SUBST(MOZ_DIRECTSHOW)
 AC_SUBST(MOZ_ANDROID_OMX)
 AC_SUBST(MOZ_APPLEMEDIA)
 AC_SUBST(MOZ_OMX_PLUGIN)
 AC_SUBST(MOZ_VPX_ERROR_CONCEALMENT)
 AC_SUBST(MOZ_VPX)
 AC_SUBST(VPX_AS)
-AC_SUBST(VPX_ASFLAGS)
-AC_SUBST(VPX_DASH_C_FLAG)
+AC_SUBST_LIST(VPX_ASFLAGS)
 AC_SUBST(VPX_AS_CONVERSION)
 AC_SUBST(VPX_ASM_SUFFIX)
 AC_SUBST(VPX_X86_ASM)
 AC_SUBST(VPX_ARM_ASM)
 AC_SUBST(VPX_NEED_OBJ_INT_EXTRACT)
 AC_SUBST(MOZ_INSTRUMENT_EVENT_LOOP)
 AC_SUBST(MOZ_CODE_COVERAGE)
 AC_SUBST(LIBJPEG_TURBO_AS)
-AC_SUBST(LIBJPEG_TURBO_ASFLAGS)
+AC_SUBST_LIST(LIBJPEG_TURBO_ASFLAGS)
 AC_SUBST(LIBJPEG_TURBO_X86_ASM)
 AC_SUBST(LIBJPEG_TURBO_X64_ASM)
 AC_SUBST(LIBJPEG_TURBO_ARM_ASM)
 AC_SUBST(LIBJPEG_TURBO_ARM64_ASM)
 AC_SUBST(LIBJPEG_TURBO_MIPS_ASM)
 
 AC_SUBST(MOZ_PACKAGE_JSSHELL)
 AC_SUBST(MOZ_FOLD_LIBS)

--- a/media/libjpeg/Makefile.in
+++ b/media/libjpeg/Makefile.in
@@ -1,16 +1,10 @@
 #
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 AS=$(LIBJPEG_TURBO_AS)
 ASM_SUFFIX=asm
-ASFLAGS=$(LIBJPEG_TURBO_ASFLAGS) -I$(topsrcdir)/media/libjpeg/simd/
-
-ifeq ($(AS),yasm)
-  # yasm doesn't like -c
-  AS_DASH_C_FLAG=
-endif
 
 include $(topsrcdir)/config/rules.mk
 

--- a/media/libjpeg/moz.build
+++ b/media/libjpeg/moz.build
@@ -145,13 +145,16 @@ elif CONFIG['LIBJPEG_TURBO_X86_ASM']:
     SOURCES += [
         'simd/jsimd_i386.c',
     ]
 else: # No SIMD support?
     SOURCES += [
         'jsimd_none.c',
     ]
 
+ASFLAGS += CONFIG['LIBJPEG_TURBO_ASFLAGS']
+ASFLAGS += ['-I%s/media/libjpeg/simd/' % TOPSRCDIR]
+
 if CONFIG['GKMEDIAS_SHARED_LIBRARY']:
     NO_VISIBILITY_FLAGS = True
 
 FINAL_LIBRARY = 'gkmedias'
 

--- a/media/libopus/Makefile.in
+++ b/media/libopus/Makefile.in
@@ -2,20 +2,16 @@
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 include $(topsrcdir)/config/rules.mk
 
 ifdef GNU_AS
 ifeq ($(CPU_ARCH),arm)
 
-# These flags are a lie; they're just used to enable the requisite
-# opcodes; actual arch detection is done at runtime.
-ASFLAGS = -march=armv7-a -mfpu=neon
-
 celt_pitch_xcorr_arm-gnu.$(ASM_SUFFIX): celt/arm/armopts-gnu.S
 
 # armopts needs a specific rule, because arm2gnu.pl will always add the .S
 # suffix when translating the files that include it.
 celt/arm/armopts-gnu.S: celt/arm/armopts.s $(call mkdir_deps,celt/arm) $(GLOBAL_DEPS)
 	$(PERL) $(srcdir)/celt/arm/arm2gnu.pl < $< > $@
 # For all others, we can use an implicit rule with the configured $(ASM_SUFFIX).
 %-gnu.$(ASM_SUFFIX): celt/arm/%.s $(GLOBAL_DEPS)

--- a/media/libopus/moz.build
+++ b/media/libopus/moz.build
@@ -89,20 +89,26 @@ else:
 
 if CONFIG['CPU_ARCH'] == 'arm' and CONFIG['GNU_AS']:
     SOURCES += celt_sources_arm
     GENERATED_SOURCES += [ '%s.%s' % (f, CONFIG['ASM_SUFFIX']) for f in [
         'celt_pitch_xcorr_arm-gnu',
     ]]
     # -Os is significantly slower, enable -O3 unless optimization is disabled
     if CONFIG['MOZ_OPTIMIZE']:
-      CFLAGS += [
-        '-O3',
-      ]
-      CXXFLAGS += [
-        '-O3',
-      ]
+        CFLAGS += [
+            '-O3',
+        ]
+        CXXFLAGS += [
+            '-O3',
+        ]
+    # These flags are a lie; they're just used to enable the requisite
+    # opcodes; actual arch detection is done at runtime.
+    ASFLAGS += [
+        '-march=armv7-a',
+        '-mfpu=neon',
+    ]
 
 # Suppress warnings in third-party code.
 if CONFIG['GNU_CC']:
     CFLAGS += ['-Wno-declaration-after-statement']
     if CONFIG['CLANG_CXX']:
         CFLAGS += ['-Wno-\#pragma-messages']

--- a/media/libtheora/Makefile.in
+++ b/media/libtheora/Makefile.in
@@ -1,20 +1,16 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 
 ifdef GNU_AS
 ifeq ($(CPU_ARCH),arm)
 
-# These flags are a lie; they're just used to enable the requisite
-# opcodes; actual arch detection is done at runtime.
-ASFLAGS = -march=armv7-a -mfpu=neon
-
 armfrag-gnu.$(ASM_SUFFIX): armopts-gnu.S
 armidct-gnu.$(ASM_SUFFIX): armopts-gnu.S
 armloop-gnu.$(ASM_SUFFIX): armopts-gnu.S
 
 # armopts needs a specific rule, because arm2gnu.pl will always add the .S
 # suffix when translating the files that include it.
 armopts-gnu.S: lib/arm/armopts.s
 	$(PERL) $(srcdir)/lib/arm/arm2gnu.pl < $< > $@

--- a/media/libtheora/moz.build
+++ b/media/libtheora/moz.build
@@ -40,24 +40,16 @@ UNIFIED_SOURCES += [
     'lib/info.c',
     'lib/internal.c',
     'lib/quant.c',
     'lib/state.c',
 ]
 
 LOCAL_INCLUDES += ['include']
 
-if CONFIG['CPU_ARCH'] == 'arm' and CONFIG['GNU_AS']:
-    GENERATED_SOURCES += [ '%s.%s' % (f, CONFIG['ASM_SUFFIX']) for f in [
-        'armbits-gnu',
-        'armfrag-gnu',
-        'armidct-gnu',
-        'armloop-gnu',
-    ]]
-
 if '86' in CONFIG['OS_TEST']:
     if CONFIG['OS_ARCH'] != 'SunOS':
         if CONFIG['OS_ARCH'] != 'WINNT' or CONFIG['OS_TEST'] != 'x86_64':
             DEFINES['OC_X86_ASM'] = True
             if '64' in CONFIG['OS_TEST']:
                 DEFINES['OC_X86_64_ASM'] = True
     if CONFIG['_MSC_VER']:
         if '64' not in CONFIG['OS_TEST']:
@@ -89,8 +81,21 @@ if CONFIG['GNU_AS']:
                     'OC_ARM_ASM_MEDIA',
                     'OC_ARM_ASM_NEON'):
             DEFINES[var] = True
         # The Android NDK doesn't pre-define anything to indicate the OS it's
         # on, so do it for them.
         if CONFIG['OS_TARGET'] == 'Android':
             DEFINES['__linux__'] = True
 
+        GENERATED_SOURCES += [ '%s.%s' % (f, CONFIG['ASM_SUFFIX']) for f in [
+            'armbits-gnu',
+            'armfrag-gnu',
+            'armidct-gnu',
+            'armloop-gnu',
+        ]]
+
+        # These flags are a lie; they're just used to enable the requisite
+        # opcodes; actual arch detection is done at runtime.
+        ASFLAGS += [
+            '-march=armv7-a',
+            '-mfpu=neon',
+        ]

--- a/media/libvpx/Makefile.in
+++ b/media/libvpx/Makefile.in
@@ -1,24 +1,17 @@
 # This Source Code Form is subject to the terms of the Mozilla Public
 # License, v. 2.0. If a copy of the MPL was not distributed with this
 # file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
 # Set up the libvpx assembler config.
 
 AS=$(VPX_AS)
-ASFLAGS=$(VPX_ASFLAGS) -I. -I$(topsrcdir)/media/libvpx/ -I$(topsrcdir)/media/libvpx/vpx_ports/
-AS_DASH_C_FLAG=$(VPX_DASH_C_FLAG)
 ASM_SUFFIX=$(VPX_ASM_SUFFIX)
 
-ifeq ($(OS_TARGET),Android)
-  # For LIBVPX_RAND
-  ASFLAGS += -D__ANDROID__
-endif
-
 ifdef VPX_ARM_ASM
 # Building on an ARM platform with a supported assembler, include
 # the optimized assembly in the build.
 
 ifeq ($(OS_TARGET),Android)
 # For cpu-features.h
 LOCAL_INCLUDES += -I$(ANDROID_NDK)/sources/android/cpufeatures
 endif

--- a/media/libvpx/moz.build
+++ b/media/libvpx/moz.build
@@ -100,8 +100,21 @@ if CONFIG['CLANG_CL'] or not CONFIG['_MS
             if 'sse4' in f:
                 SOURCES[f].flags += ['-msse4.1']
             if 'avx2' in f:
                 SOURCES[f].flags += ['-mavx2']
 
 # Suppress warnings in third-party code.
 if CONFIG['GNU_CC']:
     CFLAGS += ['-Wno-sign-compare']
+
+ASFLAGS += CONFIG['VPX_ASFLAGS']
+ASFLAGS += [
+    '-I.',
+    '-I%s/media/libvpx/' % TOPSRCDIR,
+    '-I%s/media/libvpx/vpx_ports/' % TOPSRCDIR,
+]
+
+if CONFIG['OS_TARGET'] == 'Android':
+    # For LIBVPX_RAND
+    ASFLAGS += [
+        '-D__ANDROID__'
+    ]

--- a/python/mozbuild/mozbuild/backend/recursivemake.py
+++ b/python/mozbuild/mozbuild/backend/recursivemake.py
@@ -68,16 +68,17 @@ from ..util import (
     ensureParentDir,
     FileAvoidWrite,
 )
 from ..makeutil import Makefile
 
 MOZBUILD_VARIABLES = [
     'ANDROID_GENERATED_RESFILES',
     'ANDROID_RES_DIRS',
+    'ASFLAGS',
     'CMSRCS',
     'CMMSRCS',
     'CPP_UNIT_TESTS',
     'DIRS',
     'EXTRA_DSO_LDOPTS',
     'EXTRA_JS_MODULES',
     'EXTRA_PP_COMPONENTS',
     'EXTRA_PP_JS_MODULES',

--- a/python/mozbuild/mozbuild/frontend/context.py
+++ b/python/mozbuild/mozbuild/frontend/context.py
@@ -1357,16 +1357,25 @@ VARIABLES = {
         """Flags passed to the Objective-C++ compiler for all of the
            Objective-C++ source files declared in this directory.
 
            Note that the ordering of flags matters here; these flags will be
            added to the compiler's command line in the same order as they
            appear in the moz.build file.
         """, None),
 
+    'ASFLAGS': (List, list,
+        """Flags passed to the assembler for all of the assembly source files
+           declared in this directory.
+
+           Note that the ordering of flags matters here; these flags will be
+           added to the assembler's command line in the same order as they
+           appear in the moz.build file.
+        """, None),
+
     'LDFLAGS': (List, list,
         """Flags passed to the linker when linking all of the libraries and
            executables declared in this directory.
 
            Note that the ordering of flags matters here; these flags will be
            added to the linker's command line in the same order as they
            appear in the moz.build file.
         """, None),

--- a/python/mozbuild/mozbuild/frontend/emitter.py
+++ b/python/mozbuild/mozbuild/frontend/emitter.py
@@ -577,17 +577,18 @@ class TreeMetadataEmitter(LoggingMixin):
                 passthru.variables[v] = context[v]
 
         if context.config.substs.get('OS_TARGET') == 'WINNT' and \
                 context['DELAYLOAD_DLLS']:
             context['LDFLAGS'].extend([('-DELAYLOAD:%s' % dll)
                 for dll in context['DELAYLOAD_DLLS']])
             context['OS_LIBS'].append('delayimp')
 
-        for v in ['CFLAGS', 'CXXFLAGS', 'CMFLAGS', 'CMMFLAGS', 'LDFLAGS']:
+        for v in ['CFLAGS', 'CXXFLAGS', 'CMFLAGS', 'CMMFLAGS', 'ASFLAGS',
+                  'LDFLAGS']:
             if v in context and context[v]:
                 passthru.variables['MOZBUILD_' + v] = context[v]
 
         # NO_VISIBILITY_FLAGS is slightly different
         if context['NO_VISIBILITY_FLAGS']:
             passthru.variables['VISIBILITY_FLAGS'] = ''
 
         for obj in self._process_sources(context, passthru):

--- a/xpcom/reflect/xptcall/md/unix/Makefile.in
+++ b/xpcom/reflect/xptcall/md/unix/Makefile.in
@@ -27,50 +27,26 @@ else
 # #434190 optimized builds crash
 CXXFLAGS 	:=  $(filter-out $(MOZ_OPTIMIZE_FLAGS), $(CXXFLAGS))
 endif
 endif
 endif
 
 
 ######################################################################
-# M68k
-######################################################################
-
-ifeq ($(OS_ARCH),Linux)
-ifneq (,$(findstring mips, $(OS_TEST)))
-ASFLAGS		+= -I$(DIST)/include
-endif
-endif
-
-######################################################################
 # PowerPC
 ######################################################################
 #
 # AIX/PPC
 #
 ifeq ($(OS_ARCH),AIX)
 # #24617  Building the CPP's (CXX) optimized causes a crash
 CXXFLAGS 	:=  $(filter-out $(MOZ_OPTIMIZE_FLAGS), $(CXXFLAGS))
 endif
 
-######################################################################
-# SPARC
-######################################################################
-#
-# Solaris/SPARC
-#
-ifeq ($(OS_ARCH),SunOS)
-ifneq (86,$(findstring 86,$(OS_TEST)))
-ifdef HAVE_64BIT_BUILD
-ASFLAGS		+= -xarch=v9
-endif
-endif
-endif
-
 include $(topsrcdir)/config/rules.mk
 
 ifeq ($(OS_ARCH),Linux)
 ifneq (,$(findstring mips, $(OS_TEST)))
 xptcstubs_asm_mips.o: $(DIST)/include/xptcstubsdef.inc
 endif
 endif
 
@@ -93,15 +69,11 @@ ifeq ($(OS_ARCH),SunOS)
 ifeq (86,$(findstring 86,$(OS_TEST)))
 ifndef GNU_CC
 xptcstubsdef_asm.solx86: $(DIST)/include/xptcstubsdef.inc
 	sed \
 	  -e 's/^\(STUB_ENTRY\)(\([0-9]\))/\11\(\2\)/' \
 	  -e 's/^\(STUB_ENTRY\)(\([0-9][0-9]\))/\12\(\2\)/' \
 	  -e 's/^\(STUB_ENTRY\)(\([0-9][0-9][0-9]\))/\13\(\2\)/' \
 	$(DIST)/include/xptcstubsdef.inc > $@
-ifeq (x86_64,$(OS_TEST))
-ASFLAGS += -xarch=amd64
-endif
-
 endif
 endif
 endif

--- a/xpcom/reflect/xptcall/md/unix/moz.build
+++ b/xpcom/reflect/xptcall/md/unix/moz.build
@@ -54,16 +54,17 @@ if CONFIG['OS_ARCH'] == 'SunOS' and '86'
     ]
     if CONFIG['OS_TEST'] == 'x86_64':
         if CONFIG['GNU_CC']:
             SOURCES += [
                 'xptcinvoke_x86_64_unix.cpp',
                 'xptcstubs_x86_64_linux.cpp'
             ]
         else:
+            ASFLAGS += ['-xarch=amd64']
             SOURCES += [
                 'xptcinvoke_x86_64_solaris.cpp',
                 'xptcstubs_asm_x86_64_solaris_SUNW.s',
                 'xptcstubs_x86_64_solaris.cpp',
             ]
     else:
         if CONFIG['GNU_CC']:
             SOURCES += [
@@ -266,16 +267,17 @@ if CONFIG['OS_ARCH'] in ('OpenBSD', 'Fre
         'xptcinvoke_asm_sparc64_openbsd.s',
         'xptcinvoke_sparc64_openbsd.cpp',
         'xptcstubs_asm_sparc64_openbsd.s',
         'xptcstubs_sparc64_openbsd.cpp',
     ]
 
 if CONFIG['OS_ARCH'] == 'SunOS' and '86' not in CONFIG['OS_TEST']:
     if CONFIG['HAVE_64BIT_BUILD']:
+        ASFLAGS += ['-xarch=v9']
         SOURCES += [
             'xptcinvoke_sparcv9_solaris.cpp',
             'xptcstubs_sparcv9_solaris.cpp',
         ]
     else:
         SOURCES += [
             'xptcinvoke_sparc_solaris.cpp',
             'xptcstubs_sparc_solaris.cpp',