Fix Noise compiled under clang >= 3.7.x with -O2 or higher

kwolekr · kwolekr · commit d198e420ec54 · 2015-11-01T11:16:18.000-05:00
When compiled with optimizations, the most recent versions of clang seem
to 'optimize' out a crucial "and %reg, 0x7FFFFFFF" instruction in noise2d(),
probably because it somehow assumed the variable n would never become greater
than that amount.
Indeed, signed integer underflow is undefined behavior in C and C++, so while
this optimization is "correct" in that sense, it breaks lots of existing code.
Solved by changing n to an unsigned type, making behavior well-defined.
diff --git a/src/noise.cpp b/src/noise.cpp
@@ -158,21 +158,21 @@ s32 PcgRandom::randNormalDist(s32 min, s32 max, int num_trials)
 
 float noise2d(int x, int y, int seed)
 {
-	int n = (NOISE_MAGIC_X * x + NOISE_MAGIC_Y * y
+	unsigned int n = (NOISE_MAGIC_X * x + NOISE_MAGIC_Y * y
 			+ NOISE_MAGIC_SEED * seed) & 0x7fffffff;
 	n = (n >> 13) ^ n;
 	n = (n * (n * n * 60493 + 19990303) + 1376312589) & 0x7fffffff;
-	return 1.f - (float)n / 0x40000000;
+	return 1.f - (float)(int)n / 0x40000000;
 }
 
 
 float noise3d(int x, int y, int z, int seed)
 {
-	int n = (NOISE_MAGIC_X * x + NOISE_MAGIC_Y * y + NOISE_MAGIC_Z * z
+	unsigned int n = (NOISE_MAGIC_X * x + NOISE_MAGIC_Y * y + NOISE_MAGIC_Z * z
 			+ NOISE_MAGIC_SEED * seed) & 0x7fffffff;
 	n = (n >> 13) ^ n;
 	n = (n * (n * n * 60493 + 19990303) + 1376312589) & 0x7fffffff;
-	return 1.f - (float)n / 0x40000000;
+	return 1.f - (float)(int)n / 0x40000000;
 }
 
 

Original file line number	Diff line number	Diff line change
`@@ -158,21 +158,21 @@ s32 PcgRandom::randNormalDist(s32 min, s32 max, int num_trials)`
`158`	`158`
`159`	`159`	`float noise2d(int x, int y, int seed)`
`160`	`160`	`{`
`161`		`- int n = (NOISE_MAGIC_X * x + NOISE_MAGIC_Y * y`
	`161`	`+ unsigned int n = (NOISE_MAGIC_X * x + NOISE_MAGIC_Y * y`
`162`	`162`	`+ NOISE_MAGIC_SEED * seed) & 0x7fffffff;`
`163`	`163`	`n = (n >> 13) ^ n;`
`164`	`164`	`n = (n * (n * n * 60493 + 19990303) + 1376312589) & 0x7fffffff;`
`165`		`- return 1.f - (float)n / 0x40000000;`
	`165`	`+ return 1.f - (float)(int)n / 0x40000000;`
`166`	`166`	`}`
`167`	`167`
`168`	`168`
`169`	`169`	`float noise3d(int x, int y, int z, int seed)`
`170`	`170`	`{`
`171`		`- int n = (NOISE_MAGIC_X * x + NOISE_MAGIC_Y * y + NOISE_MAGIC_Z * z`
	`171`	`+ unsigned int n = (NOISE_MAGIC_X * x + NOISE_MAGIC_Y * y + NOISE_MAGIC_Z * z`
`172`	`172`	`+ NOISE_MAGIC_SEED * seed) & 0x7fffffff;`
`173`	`173`	`n = (n >> 13) ^ n;`
`174`	`174`	`n = (n * (n * n * 60493 + 19990303) + 1376312589) & 0x7fffffff;`
`175`		`- return 1.f - (float)n / 0x40000000;`
	`175`	`+ return 1.f - (float)(int)n / 0x40000000;`
`176`	`176`	`}`
`177`	`177`
`178`	`178`