Commit `878b7b80c1`

Koki Ueha <jeamsblue@gmail.com>

2025-06-15 03:05:58

libc: Prevent FCSEL instruction from being used to avoid raising an unintended exception

If you write an if expression in mem.doNotOptimizeAway like doNotOptimizeAway(if (ix < 0x00100000) x / 0x1p120 else x + 0x1p120);, FCSEL instruction is used on AArch64. FCSEL instruction selects one of the two registers according to the condition and copies its value. In this example, `x / 0x1p120` and `x + 0x1p120` are expressions that raise different floating-point exceptions. However, since both are actually evaluated before the FCSEL instruction, the exception not intended by the programmer may also be raised. To prevent FCSEL instruction from being used here, this commit splits doNotOptimizeAway in two.

master

1 parent 3ce8d19

Changed files (3)

lib

compiler_rt

sin.zig

sincos.zig

tan.zig

@@ -49,7 +49,13 @@ pub fn sinf(x: f32) callconv(.c) f32 {
     if (ix <= 0x3f490fda) { // |x| ~<= pi/4
         if (ix < 0x39800000) { // |x| < 2**-12
             // raise inexact if x!=0 and underflow if subnormal
-            if (common.want_float_exceptions) mem.doNotOptimizeAway(if (ix < 0x00800000) x / 0x1p120 else x + 0x1p120);
+            if (common.want_float_exceptions) {
+                if (ix < 0x00800000) {
+                    mem.doNotOptimizeAway(x / 0x1p120);
+                } else {
+                    mem.doNotOptimizeAway(x + 0x1p120);
+                }
+            }
             return x;
         }
         return trig.__sindf(x);
@@ -98,7 +104,13 @@ pub fn sin(x: f64) callconv(.c) f64 {
     if (ix <= 0x3fe921fb) {
         if (ix < 0x3e500000) { // |x| < 2**-26
             // raise inexact if x != 0 and underflow if subnormal
-            if (common.want_float_exceptions) mem.doNotOptimizeAway(if (ix < 0x00100000) x / 0x1p120 else x + 0x1p120);
+            if (common.want_float_exceptions) {
+                if (ix < 0x00100000) {
+                    mem.doNotOptimizeAway(x / 0x1p120);
+                } else {
+                    mem.doNotOptimizeAway(x + 0x1p120);
+                }
+            }
             return x;
         }
         return trig.__sin(x, 0.0, 0);

@@ -46,7 +46,13 @@ pub fn sincosf(x: f32, r_sin: *f32, r_cos: *f32) callconv(.c) void {
         // |x| < 2**-12
         if (ix < 0x39800000) {
             // raise inexact if x!=0 and underflow if subnormal
-            if (common.want_float_exceptions) mem.doNotOptimizeAway(if (ix < 0x00100000) x / 0x1p120 else x + 0x1p120);
+            if (common.want_float_exceptions) {
+                if (ix < 0x00100000) {
+                    mem.doNotOptimizeAway(x / 0x1p120);
+                } else {
+                    mem.doNotOptimizeAway(x + 0x1p120);
+                }
+            }
             r_sin.* = x;
             r_cos.* = 1.0;
             return;
@@ -134,7 +140,13 @@ pub fn sincos(x: f64, r_sin: *f64, r_cos: *f64) callconv(.c) void {
         // if |x| < 2**-27 * sqrt(2)
         if (ix < 0x3e46a09e) {
             // raise inexact if x != 0 and underflow if subnormal
-            if (common.want_float_exceptions) mem.doNotOptimizeAway(if (ix < 0x00100000) x / 0x1p120 else x + 0x1p120);
+            if (common.want_float_exceptions) {
+                if (ix < 0x00100000) {
+                    mem.doNotOptimizeAway(x / 0x1p120);
+                } else {
+                    mem.doNotOptimizeAway(x + 0x1p120);
+                }
+            }
             r_sin.* = x;
             r_cos.* = 1.0;
             return;

@@ -51,7 +51,13 @@ pub fn tanf(x: f32) callconv(.c) f32 {
     if (ix <= 0x3f490fda) { // |x| ~<= pi/4
         if (ix < 0x39800000) { // |x| < 2**-12
             // raise inexact if x!=0 and underflow if subnormal
-            if (common.want_float_exceptions) mem.doNotOptimizeAway(if (ix < 0x00800000) x / 0x1p120 else x + 0x1p120);
+            if (common.want_float_exceptions) {
+                if (ix < 0x00800000) {
+                    mem.doNotOptimizeAway(x / 0x1p120);
+                } else {
+                    mem.doNotOptimizeAway(x + 0x1p120);
+                }
+            }
             return x;
         }
         return kernel.__tandf(x, false);
@@ -89,7 +95,13 @@ pub fn tan(x: f64) callconv(.c) f64 {
     if (ix <= 0x3fe921fb) {
         if (ix < 0x3e400000) { // |x| < 2**-27
             // raise inexact if x!=0 and underflow if subnormal
-            if (common.want_float_exceptions) mem.doNotOptimizeAway(if (ix < 0x00100000) x / 0x1p120 else x + 0x1p120);
+            if (common.want_float_exceptions) {
+                if (ix < 0x00100000) {
+                    mem.doNotOptimizeAway(x / 0x1p120);
+                } else {
+                    mem.doNotOptimizeAway(x + 0x1p120);
+                }
+            }
             return x;
         }
         return kernel.__tan(x, 0.0, false);

Commit 878b7b80c1

Commit `878b7b80c1`