diff --git a/simd/powerpc/jccolext-altivec.c b/simd/powerpc/jccolext-altivec.c
index 170f90ff..a51b4be7 100644
--- a/simd/powerpc/jccolext-altivec.c
+++ b/simd/powerpc/jccolext-altivec.c
@@ -1,7 +1,7 @@
 /*
  * AltiVec optimizations for libjpeg-turbo
  *
- * Copyright (C) 2014-2015, D. R. Commander.  All Rights Reserved.
+ * Copyright (C) 2014-2015, 2024, D. R. Commander.  All Rights Reserved.
  * Copyright (C) 2014, Jay Foad.  All Rights Reserved.
  *
  * This software is provided 'as-is', without any express or implied
@@ -30,17 +30,17 @@ void jsimd_rgb_ycc_convert_altivec(JDIMENSION img_width, JSAMPARRAY input_buf,
 {
   JSAMPROW inptr, outptr0, outptr1, outptr2;
   int pitch = img_width * RGB_PIXELSIZE, num_cols;
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
   int offset;
 #endif
   unsigned char __attribute__((aligned(16))) tmpbuf[RGB_PIXELSIZE * 16];
 
   __vector unsigned char rgb0, rgb1 = { 0 }, rgb2 = { 0 },
     rgbg0, rgbg1, rgbg2, rgbg3, y, cb, cr;
-#if __BIG_ENDIAN__ || RGB_PIXELSIZE == 4
+#if defined(__BIG_ENDIAN__) || RGB_PIXELSIZE == 4
   __vector unsigned char rgb3 = { 0 };
 #endif
-#if __BIG_ENDIAN__ && RGB_PIXELSIZE == 4
+#if defined(__BIG_ENDIAN__) && RGB_PIXELSIZE == 4
   __vector unsigned char rgb4 = { 0 };
 #endif
   __vector short rg0, rg1, rg2, rg3, bg0, bg1, bg2, bg3;
@@ -56,7 +56,7 @@ void jsimd_rgb_ycc_convert_altivec(JDIMENSION img_width, JSAMPARRAY input_buf,
   __vector int pd_onehalf = { __4X(ONE_HALF) },
     pd_onehalfm1_cj = { __4X(ONE_HALF - 1 + (CENTERJSAMPLE << SCALEBITS)) };
   __vector unsigned char pb_zero = { __16X(0) },
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
     shift_pack_index =
       {  0,  1,  4,  5,  8,  9, 12, 13, 16, 17, 20, 21, 24, 25, 28, 29 };
 #else
@@ -75,7 +75,7 @@ void jsimd_rgb_ycc_convert_altivec(JDIMENSION img_width, JSAMPARRAY input_buf,
          num_cols -= RGB_PIXELSIZE * 16, inptr += RGB_PIXELSIZE * 16,
          outptr0 += 16, outptr1 += 16, outptr2 += 16) {
 
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
       /* Load 16 pixels == 48 or 64 bytes */
       offset = (size_t)inptr & 15;
       if (offset) {
@@ -141,7 +141,7 @@ void jsimd_rgb_ycc_convert_altivec(JDIMENSION img_width, JSAMPARRAY input_buf,
             rgb3 = VEC_LD(48, inptr);
 #endif
         }
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
       }
 #endif
 
diff --git a/simd/powerpc/jcgryext-altivec.c b/simd/powerpc/jcgryext-altivec.c
index b280cbbd..550e239a 100644
--- a/simd/powerpc/jcgryext-altivec.c
+++ b/simd/powerpc/jcgryext-altivec.c
@@ -1,7 +1,7 @@
 /*
  * AltiVec optimizations for libjpeg-turbo
  *
- * Copyright (C) 2014-2015, D. R. Commander.  All Rights Reserved.
+ * Copyright (C) 2014-2015, 2024, D. R. Commander.  All Rights Reserved.
  * Copyright (C) 2014, Jay Foad.  All Rights Reserved.
  *
  * This software is provided 'as-is', without any express or implied
@@ -30,17 +30,17 @@ void jsimd_rgb_gray_convert_altivec(JDIMENSION img_width, JSAMPARRAY input_buf,
 {
   JSAMPROW inptr, outptr;
   int pitch = img_width * RGB_PIXELSIZE, num_cols;
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
   int offset;
   unsigned char __attribute__((aligned(16))) tmpbuf[RGB_PIXELSIZE * 16];
 #endif
 
   __vector unsigned char rgb0, rgb1 = { 0 }, rgb2 = { 0 },
     rgbg0, rgbg1, rgbg2, rgbg3, y;
-#if __BIG_ENDIAN__ || RGB_PIXELSIZE == 4
+#if defined(__BIG_ENDIAN__) || RGB_PIXELSIZE == 4
   __vector unsigned char rgb3 = { 0 };
 #endif
-#if __BIG_ENDIAN__ && RGB_PIXELSIZE == 4
+#if defined(__BIG_ENDIAN__) && RGB_PIXELSIZE == 4
   __vector unsigned char rgb4 = { 0 };
 #endif
   __vector short rg0, rg1, rg2, rg3, bg0, bg1, bg2, bg3;
@@ -52,7 +52,7 @@ void jsimd_rgb_gray_convert_altivec(JDIMENSION img_width, JSAMPARRAY input_buf,
     pw_f0114_f0250 = { __4X2(F_0_114, F_0_250) };
   __vector int pd_onehalf = { __4X(ONE_HALF) };
   __vector unsigned char pb_zero = { __16X(0) },
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
     shift_pack_index =
       { 0, 1, 4, 5,  8,  9, 12, 13, 16, 17, 20, 21, 24, 25, 28, 29 };
 #else
@@ -69,7 +69,7 @@ void jsimd_rgb_gray_convert_altivec(JDIMENSION img_width, JSAMPARRAY input_buf,
          num_cols -= RGB_PIXELSIZE * 16, inptr += RGB_PIXELSIZE * 16,
          outptr += 16) {
 
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
       /* Load 16 pixels == 48 or 64 bytes */
       offset = (size_t)inptr & 15;
       if (offset) {
diff --git a/simd/powerpc/jcsample-altivec.c b/simd/powerpc/jcsample-altivec.c
index 6e25b8db..6bda03a8 100644
--- a/simd/powerpc/jcsample-altivec.c
+++ b/simd/powerpc/jcsample-altivec.c
@@ -1,7 +1,7 @@
 /*
  * AltiVec optimizations for libjpeg-turbo
  *
- * Copyright (C) 2015, D. R. Commander.  All Rights Reserved.
+ * Copyright (C) 2015, 2024, D. R. Commander.  All Rights Reserved.
  *
  * This software is provided 'as-is', without any express or implied
  * warranty.  In no event will the authors be held liable for any damages
@@ -50,7 +50,7 @@ void jsimd_h2v1_downsample_altivec(JDIMENSION image_width,
   expand_right_edge(input_data, max_v_samp_factor, image_width,
                     output_cols * 2);
 
-  for (outrow = 0; outrow < v_samp_factor; outrow++) {
+  for (outrow = 0; outrow < (int)v_samp_factor; outrow++) {
     outptr = output_data[outrow];
     inptr = input_data[outrow];
 
@@ -107,7 +107,7 @@ jsimd_h2v2_downsample_altivec(JDIMENSION image_width, int max_v_samp_factor,
   expand_right_edge(input_data, max_v_samp_factor, image_width,
                     output_cols * 2);
 
-  for (inrow = 0, outrow = 0; outrow < v_samp_factor;
+  for (inrow = 0, outrow = 0; outrow < (int)v_samp_factor;
        inrow += 2, outrow++) {
 
     inptr0 = input_data[inrow];
diff --git a/simd/powerpc/jdcolext-altivec.c b/simd/powerpc/jdcolext-altivec.c
index 68d52bd8..7e3a2049 100644
--- a/simd/powerpc/jdcolext-altivec.c
+++ b/simd/powerpc/jdcolext-altivec.c
@@ -1,7 +1,7 @@
 /*
  * AltiVec optimizations for libjpeg-turbo
  *
- * Copyright (C) 2015, D. R. Commander.  All Rights Reserved.
+ * Copyright (C) 2015, 2024, D. R. Commander.  All Rights Reserved.
  *
  * This software is provided 'as-is', without any express or implied
  * warranty.  In no event will the authors be held liable for any damages
@@ -29,14 +29,14 @@ void jsimd_ycc_rgb_convert_altivec(JDIMENSION out_width, JSAMPIMAGE input_buf,
 {
   JSAMPROW outptr, inptr0, inptr1, inptr2;
   int pitch = out_width * RGB_PIXELSIZE, num_cols;
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
   int offset;
 #endif
   unsigned char __attribute__((aligned(16))) tmpbuf[RGB_PIXELSIZE * 16];
 
   __vector unsigned char rgb0, rgb1, rgb2, rgbx0, rgbx1, rgbx2, rgbx3,
     y, cb, cr;
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
   __vector unsigned char edgel, edgeh, edges, out0, out1, out2, out3;
 #if RGB_PIXELSIZE == 4
   __vector unsigned char out4;
@@ -60,7 +60,7 @@ void jsimd_ycc_rgb_convert_altivec(JDIMENSION out_width, JSAMPIMAGE input_buf,
     pw_cj = { __8X(CENTERJSAMPLE) };
   __vector int pd_onehalf = { __4X(ONE_HALF) };
   __vector unsigned char pb_zero = { __16X(0) },
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
     shift_pack_index =
       {  0,  1,  4,  5,  8,  9, 12, 13, 16, 17, 20, 21, 24, 25, 28, 29 };
 #else
@@ -196,7 +196,7 @@ void jsimd_ycc_rgb_convert_altivec(JDIMENSION out_width, JSAMPIMAGE input_buf,
       rgb3 = vec_perm(rgbx3, rgbx3, (__vector unsigned char)RGB_INDEX);
 #endif
 
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
       offset = (size_t)outptr & 15;
       if (offset) {
         __vector unsigned char unaligned_shift_index;
@@ -268,7 +268,7 @@ void jsimd_ycc_rgb_convert_altivec(JDIMENSION out_width, JSAMPIMAGE input_buf,
             VEC_ST(rgb3, 48, outptr);
 #endif
         }
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
       }
 #endif
     }
diff --git a/simd/powerpc/jdmrgext-altivec.c b/simd/powerpc/jdmrgext-altivec.c
index 40f02c33..167b7380 100644
--- a/simd/powerpc/jdmrgext-altivec.c
+++ b/simd/powerpc/jdmrgext-altivec.c
@@ -1,7 +1,7 @@
 /*
  * AltiVec optimizations for libjpeg-turbo
  *
- * Copyright (C) 2015, D. R. Commander.  All Rights Reserved.
+ * Copyright (C) 2015, 2024, D. R. Commander.  All Rights Reserved.
  *
  * This software is provided 'as-is', without any express or implied
  * warranty.  In no event will the authors be held liable for any damages
@@ -30,14 +30,14 @@ void jsimd_h2v1_merged_upsample_altivec(JDIMENSION output_width,
 {
   JSAMPROW outptr, inptr0, inptr1, inptr2;
   int pitch = output_width * RGB_PIXELSIZE, num_cols, yloop;
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
   int offset;
 #endif
   unsigned char __attribute__((aligned(16))) tmpbuf[RGB_PIXELSIZE * 16];
 
   __vector unsigned char rgb0, rgb1, rgb2, rgbx0, rgbx1, rgbx2, rgbx3,
     y, cb, cr;
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
   __vector unsigned char edgel, edgeh, edges, out0, out1, out2, out3;
 #if RGB_PIXELSIZE == 4
   __vector unsigned char out4;
@@ -62,7 +62,7 @@ void jsimd_h2v1_merged_upsample_altivec(JDIMENSION output_width,
     pw_cj = { __8X(CENTERJSAMPLE) };
   __vector int pd_onehalf = { __4X(ONE_HALF) };
   __vector unsigned char pb_zero = { __16X(0) },
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
     shift_pack_index =
       {  0,  1,  4,  5,  8,  9, 12, 13, 16, 17, 20, 21, 24, 25, 28, 29 },
     even_index =
@@ -225,7 +225,7 @@ void jsimd_h2v1_merged_upsample_altivec(JDIMENSION output_width,
       rgb3 = vec_perm(rgbx3, rgbx3, (__vector unsigned char)RGB_INDEX);
 #endif
 
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
       offset = (size_t)outptr & 15;
       if (offset) {
         __vector unsigned char unaligned_shift_index;
@@ -297,7 +297,7 @@ void jsimd_h2v1_merged_upsample_altivec(JDIMENSION output_width,
             VEC_ST(rgb3, 48, outptr);
 #endif
         }
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
       }
 #endif
     }
diff --git a/simd/powerpc/jdsample-altivec.c b/simd/powerpc/jdsample-altivec.c
index 04df0cf1..b04f830b 100644
--- a/simd/powerpc/jdsample-altivec.c
+++ b/simd/powerpc/jdsample-altivec.c
@@ -1,7 +1,7 @@
 /*
  * AltiVec optimizations for libjpeg-turbo
  *
- * Copyright (C) 2015, D. R. Commander.  All Rights Reserved.
+ * Copyright (C) 2015, 2024, D. R. Commander.  All Rights Reserved.
  *
  * This software is provided 'as-is', without any express or implied
  * warranty.  In no event will the authors be held liable for any damages
@@ -49,7 +49,7 @@ void jsimd_h2v1_fancy_upsample_altivec(int max_v_samp_factor,
       {  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16 },
     next_index_lastcol =
       {  1,  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 15 },
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
     merge_pack_index =
       {  1, 17,  3, 19,  5, 21,  7, 23,  9, 25, 11, 27, 13, 29, 15, 31 };
 #else
@@ -155,7 +155,7 @@ void jsimd_h2v2_fancy_upsample_altivec(int max_v_samp_factor,
       {  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 16, 17 },
     next_index_lastcol =
       {  2,  3,  4,  5,  6,  7,  8,  9, 10, 11, 12, 13, 14, 15, 14, 15 },
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
     merge_pack_index =
       {  1, 17,  3, 19,  5, 21,  7, 23,  9, 25, 11, 27, 13, 29, 15, 31 };
 #else
diff --git a/simd/powerpc/jidctfst-altivec.c b/simd/powerpc/jidctfst-altivec.c
index 456c6c61..06839c79 100644
--- a/simd/powerpc/jidctfst-altivec.c
+++ b/simd/powerpc/jidctfst-altivec.c
@@ -1,7 +1,7 @@
 /*
  * AltiVec optimizations for libjpeg-turbo
  *
- * Copyright (C) 2014-2015, D. R. Commander.  All Rights Reserved.
+ * Copyright (C) 2014-2015, 2024, D. R. Commander.  All Rights Reserved.
  *
  * This software is provided 'as-is', without any express or implied
  * warranty.  In no event will the authors be held liable for any damages
@@ -127,12 +127,12 @@ void jsimd_idct_ifast_altivec(void *dct_table_, JCOEFPTR coef_block,
   __vector short pw_zero = { __8X(0) },
     pw_F1414 = { __8X(F_1_414 << CONST_SHIFT) },
     pw_F1847 = { __8X(F_1_847 << CONST_SHIFT) },
-    pw_MF1613 = { __8X(-F_1_613 << CONST_SHIFT) },
+    pw_MF1613 = { __8X((short)((unsigned short)(-F_1_613) << CONST_SHIFT)) },
     pw_F1082 = { __8X(F_1_082 << CONST_SHIFT) };
   __vector unsigned short
     pre_multiply_scale_bits = { __8X(PRE_MULTIPLY_SCALE_BITS) },
     pass1_bits3 = { __8X(PASS1_BITS + 3) };
-  __vector signed char pb_centerjsamp = { __16X(CENTERJSAMPLE) };
+  __vector signed char pb_centerjsamp = { __16X((signed char)CENTERJSAMPLE) };
 
   /* Pass 1: process columns */
 
diff --git a/simd/powerpc/jidctint-altivec.c b/simd/powerpc/jidctint-altivec.c
index 60e619f1..52e37d90 100644
--- a/simd/powerpc/jidctint-altivec.c
+++ b/simd/powerpc/jidctint-altivec.c
@@ -1,7 +1,7 @@
 /*
  * AltiVec optimizations for libjpeg-turbo
  *
- * Copyright (C) 2014-2015, 2020, D. R. Commander.  All Rights Reserved.
+ * Copyright (C) 2014-2015, 2020, 2024, D. R. Commander.  All Rights Reserved.
  *
  * This software is provided 'as-is', without any express or implied
  * warranty.  In no event will the authors be held liable for any damages
@@ -241,7 +241,7 @@ void jsimd_idct_islow_altivec(void *dct_table_, JCOEFPTR coef_block,
   __vector unsigned int descale_p1 = { __4X(DESCALE_P1) },
     descale_p2 = { __4X(DESCALE_P2) },
     const_bits = { __4X(CONST_BITS) };
-  __vector signed char pb_centerjsamp = { __16X(CENTERJSAMPLE) };
+  __vector signed char pb_centerjsamp = { __16X((signed char)CENTERJSAMPLE) };
 
   /* Pass 1: process columns */
 
diff --git a/simd/powerpc/jquanti-altivec.c b/simd/powerpc/jquanti-altivec.c
index 7d6e3254..05ea13e7 100644
--- a/simd/powerpc/jquanti-altivec.c
+++ b/simd/powerpc/jquanti-altivec.c
@@ -1,7 +1,7 @@
 /*
  * AltiVec optimizations for libjpeg-turbo
  *
- * Copyright (C) 2014-2015, D. R. Commander.  All Rights Reserved.
+ * Copyright (C) 2014-2015, 2024, D. R. Commander.  All Rights Reserved.
  *
  * This software is provided 'as-is', without any express or implied
  * warranty.  In no event will the authors be held liable for any damages
@@ -29,7 +29,7 @@
  * always get the data we want by using a single vector load (although we may
  * have to permute the result.)
  */
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
 
 #define LOAD_ROW(row) { \
   elemptr = sample_data[row] + start_col; \
@@ -125,7 +125,7 @@ void jsimd_quantize_altivec(JCOEFPTR coef_block, DCTELEM *divisors,
 
   /* Constants */
   __vector unsigned short pw_word_bit_m1 = { __8X(WORD_BIT - 1) };
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
   __vector unsigned char shift_pack_index =
     {  0,  1, 16, 17,  4,  5, 20, 21,  8,  9, 24, 25, 12, 13, 28, 29 };
 #else
diff --git a/simd/powerpc/jsimd_altivec.h b/simd/powerpc/jsimd_altivec.h
index e8bdb06a..29c81964 100644
--- a/simd/powerpc/jsimd_altivec.h
+++ b/simd/powerpc/jsimd_altivec.h
@@ -81,7 +81,7 @@
 
 /* Macros to abstract big/little endian bit twiddling */
 
-#if __BIG_ENDIAN__
+#ifdef __BIG_ENDIAN__
 
 #define VEC_LD(a, b)     vec_ld(a, b)
 #define VEC_ST(a, b, c)  vec_st(a, b, c)
@@ -96,3 +96,12 @@
 #define VEC_UNPACKLU(a)  vec_mergel(a, pb_zero)
 
 #endif
+
+
+#if defined(__clang__)
+#pragma clang diagnostic ignored "-Wc99-extensions"
+#pragma clang diagnostic ignored "-Wshadow"
+#elif defined(__GNUC__)
+#pragma GCC diagnostic ignored "-Wpedantic"
+#pragma GCC diagnostic ignored "-Wshadow"
+#endif