Workaround MSVC miscompiling sgen_clz (#114786)

akoeplinger · web-flow · commit 2dc2ec995196 · 2025-04-17T19:47:50.000+02:00
After the recent VS upgrade from 17.12.5 to 17.13.2 we started seeing access violations in the mono-aot-cross.exe when targetting wasm.

We tracked it down to sgen_clz being miscompiled, we can workaround the compiler bug by switching from ternary condition to if/else.
diff --git a/src/mono/mono/sgen/sgen-array-list.h b/src/mono/mono/sgen/sgen-array-list.h
@@ -60,7 +60,10 @@ static inline guint32
 sgen_clz (guint32 x)
 {
 	gulong leading_zero_bits;
-	return _BitScanReverse (&leading_zero_bits, (gulong)x) ? 31 - leading_zero_bits : 32;
+	if (_BitScanReverse (&leading_zero_bits, (gulong)x))
+		return 31 - leading_zero_bits;
+	else
+		return 32;
 }
 #elif defined(ENABLE_MSVC_LZCNT) && defined(_MSC_VER)
 static inline guint32

Original file line number	Diff line number	Diff line change
`@@ -60,7 +60,10 @@ static inline guint32`
`60`	`60`	`sgen_clz (guint32 x)`
`61`	`61`	`{`
`62`	`62`	`gulong leading_zero_bits;`
`63`		`- return _BitScanReverse (&leading_zero_bits, (gulong)x) ? 31 - leading_zero_bits : 32;`
	`63`	`+ if (_BitScanReverse (&leading_zero_bits, (gulong)x))`
	`64`	`+ return 31 - leading_zero_bits;`
	`65`	`+ else`
	`66`	`+ return 32;`
`64`	`67`	`}`
`65`	`68`	`#elif defined(ENABLE_MSVC_LZCNT) && defined(_MSC_VER)`
`66`	`69`	`static inline guint32`