diff --git a/GNUmakefile b/GNUmakefile
index e013db84..dd87c888 100755
--- a/GNUmakefile
+++ b/GNUmakefile
@@ -339,11 +339,12 @@ ifeq ($(IS_ARMV8),1)
 endif
 
 # PowerPC and PowerPC-64
+# Altivec is available with Power4, but the library is tied to Power7 and unaligned loads/stores.
 ifneq ($(IS_PPC32)$(IS_PPC64)$(IS_AIX),000)
   # GCC and some compatibles
-  HAVE_ALTIVEC = $(shell echo | $(CXX) -x c++ $(CXXFLAGS) -maltivec -dM -E - 2>/dev/null | $(GREP) -i -c '__ALTIVEC__')
+  HAVE_ALTIVEC = $(shell echo | $(CXX) -x c++ $(CXXFLAGS) -mcpu=power7 -maltivec -dM -E - 2>/dev/null | $(GREP) -i -c '__ALTIVEC__')
   ifneq ($(HAVE_ALTIVEC),0)
-    ALTIVEC_FLAG = -maltivec
+    ALTIVEC_FLAG = -mcpu=power7 -maltivec
   endif
   # GCC and some compatibles
   HAVE_CRYPTO = $(shell echo | $(CXX) -x c++ $(CXXFLAGS) -mcpu=power8 -maltivec -dM -E - 2>/dev/null | $(GREP) -i -c -E '_ARCH_PWR8|_ARCH_PWR9|__CRYPTO')
@@ -354,9 +355,9 @@ ifneq ($(IS_PPC32)$(IS_PPC64)$(IS_AIX),000)
     ALTIVEC_FLAG = -mcpu=power8 -maltivec
   endif
   # IBM XL C/C++
-  HAVE_ALTIVEC = $(shell $(CXX) $(CXXFLAGS) -qshowmacros -qaltivec -E adhoc.cpp.proto 2>/dev/null | $(GREP) -i -c '__ALTIVEC__')
+  HAVE_ALTIVEC = $(shell $(CXX) $(CXXFLAGS) -qshowmacros -qarch=pwr7 -qaltivec -E adhoc.cpp.proto 2>/dev/null | $(GREP) -i -c '__ALTIVEC__')
   ifneq ($(HAVE_ALTIVEC),0)
-    ALTIVEC_FLAG = -qaltivec
+    ALTIVEC_FLAG = -qarch=pwr7 -qaltivec
   endif
   # IBM XL C/C++
   HAVE_CRYPTO = $(shell $(CXX) $(CXXFLAGS) -qshowmacros -qarch=pwr8 -qaltivec -E adhoc.cpp.proto 2>/dev/null | $(GREP) -i -c -E '_ARCH_PWR8|_ARCH_PWR9|__CRYPTO')
@@ -366,6 +367,10 @@ ifneq ($(IS_PPC32)$(IS_PPC64)$(IS_AIX),000)
     SHA_FLAG = -qarch=pwr8 -qaltivec
     ALTIVEC_FLAG = -qarch=pwr8 -qaltivec
   endif
+  # Fail safe to disable intrinsics on down level machines, like PowerMac G5
+  ifeq ($(ALTIVEC_FLAG),)
+    CXXFLAGS += -DCRYPTOPP_DISABLE_ALTIVEC
+  endif
 endif
 
 # IBM XL C/C++ compiler
@@ -378,10 +383,6 @@ ifeq ($(XLC_COMPILER),1)
   ifneq ($(findstring -fPIC,$(CXXFLAGS)),)
       CXXFLAGS := $(CXXFLAGS:-fPIC=-qpic)
   endif
-  # Warnings and intermittent failures on early IBM XL C/C++
-  #ifneq ($(findstring -O3,$(CXXFLAGS)),)
-  #    CXXFLAGS := $(CXXFLAGS:-O3=-O2)
-  #endif
 endif
 
 endif	# IS_X86
diff --git a/config.h b/config.h
index e106b698..de93c841 100644
--- a/config.h
+++ b/config.h
@@ -600,20 +600,21 @@ NAMESPACE_END
 
 #if (CRYPTOPP_BOOL_PPC32 || CRYPTOPP_BOOL_PPC64)
 
-// An old Apple G5 with GCC 4.01 has AltiVec.
-#if !defined(CRYPTOPP_ALTIVEC_AVAILABLE) && !defined(CRYPTOPP_DISABLE_ASM)
+// An old Apple G5 with GCC 4.01 has AltiVec, but its only Power4 or so.
+// We need Power7 or above, so the makefile defines CRYPTOPP_DISABLE_ALTIVEC.
+#if !defined(CRYPTOPP_ALTIVEC_AVAILABLE) && !defined(CRYPTOPP_DISABLE_ALTIVEC) && !defined(CRYPTOPP_DISABLE_ASM)
 # if defined(__ALTIVEC__) || (CRYPTOPP_XLC_VERSION >= 100000) || (CRYPTOPP_GCC_VERSION >= 40000)
 #  define CRYPTOPP_ALTIVEC_AVAILABLE 1
 # endif
 #endif
 
-#if !defined(CRYPTOPP_POWER8_AVAILABLE) && !defined(CRYPTOPP_DISABLE_ASM)
+#if !defined(CRYPTOPP_POWER8_AVAILABLE) && !defined(CRYPTOPP_DISABLE_ASM) && defined(CRYPTOPP_ALTIVEC_AVAILABLE)
 # if defined(_ARCH_PWR8) || (CRYPTOPP_XLC_VERSION >= 130000) || (CRYPTOPP_GCC_VERSION >= 40800)
 #  define CRYPTOPP_POWER8_AVAILABLE 1
 # endif
 #endif
 
-#if !defined(CRYPTOPP_POWER8_CRYPTO_AVAILABLE) && !defined(CRYPTOPP_DISABLE_ASM)
+#if !defined(CRYPTOPP_POWER8_CRYPTO_AVAILABLE) && !defined(CRYPTOPP_DISABLE_ASM) && defined(CRYPTOPP_ALTIVEC_AVAILABLE)
 # if defined(__CRYPTO__) || defined(_ARCH_PWR8) || (CRYPTOPP_XLC_VERSION >= 130000) || (CRYPTOPP_GCC_VERSION >= 40800)
 #  define CRYPTOPP_POWER8_AES_AVAILABLE 1
 //#  define CRYPTOPP_POWER8_SHA_AVAILABLE 1