From 6b66b67f69e2f1934800b372431b5808105dc56c Mon Sep 17 00:00:00 2001
From: Corentin Wallez <cwallez@chromium.org>
Date: Wed, 22 Jan 2020 13:05:16 +0000
Subject: [PATCH] Fix compilation of Log2(uint64_t) on MSVC

The _BitScanReverse64 intrinsic only exists when compiling for 64bit.
Replace it by two calls to _BitScanReverse (the 32bit one) when on
32bit.

Bug:
Change-Id: Ie294327ec914b0ca4a73732e4b78c1f2a08f100f
Reviewed-on: https://dawn-review.googlesource.com/c/dawn/+/15321
Commit-Queue: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Kai Ninomiya <kainino@chromium.org>
---
 src/common/Math.cpp | 24 ++++++++++++++----------
 1 file changed, 14 insertions(+), 10 deletions(-)

diff --git a/src/common/Math.cpp b/src/common/Math.cpp
index 2ae977b1e8..4471eb77aa 100644
--- a/src/common/Math.cpp
+++ b/src/common/Math.cpp
@@ -15,6 +15,7 @@
 #include "common/Math.h"
 
 #include "common/Assert.h"
+#include "common/Platform.h"
 
 #include <algorithm>
 #include <cmath>
@@ -50,28 +51,31 @@ uint32_t Log2(uint32_t value) {
 uint32_t Log2(uint64_t value) {
     ASSERT(value != 0);
 #if defined(DAWN_COMPILER_MSVC)
+#    if defined(DAWN_PLATFORM_64_BIT)
     unsigned long firstBitIndex = 0ul;
     unsigned char ret = _BitScanReverse64(&firstBitIndex, value);
     ASSERT(ret != 0);
     return firstBitIndex;
-#else
+#    else   // defined(DAWN_PLATFORM_64_BIT)
+    unsigned long firstBitIndex = 0ul;
+    if (_BitScanReverse(&firstBitIndex, value >> 32)) {
+        return firstBitIndex + 32;
+    }
+    unsigned char ret = _BitScanReverse(&firstBitIndex, value & 0xFFFFFFFF);
+    ASSERT(ret != 0);
+    return firstBitIndex;
+#    endif  // defined(DAWN_PLATFORM_64_BIT)
+#else       // defined(DAWN_COMPILER_MSVC)
     return 63 - static_cast<uint32_t>(__builtin_clzll(value));
-#endif
+#endif      // defined(DAWN_COMPILER_MSVC)
 }
 
 uint64_t NextPowerOfTwo(uint64_t n) {
-#if defined(DAWN_COMPILER_MSVC)
     if (n <= 1) {
         return 1;
     }
 
-    unsigned long firstBitIndex = 0ul;
-    unsigned char ret = _BitScanReverse64(&firstBitIndex, n - 1);
-    ASSERT(ret != 0);
-    return 1ull << (firstBitIndex + 1);
-#else
-    return n <= 1 ? 1 : 1ull << (64 - __builtin_clzll(n - 1));
-#endif
+    return 1ull << (Log2(n - 1) + 1);
 }
 
 bool IsPowerOfTwo(uint64_t n) {