From 878b7b80c15d4b9a894f6ab1d4e8753de77568ad Mon Sep 17 00:00:00 2001
From: Koki Ueha <jeamsblue@gmail.com>
Date: Sun, 15 Jun 2025 01:05:58 +0000
Subject: [PATCH] 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.
---
 lib/compiler_rt/sin.zig    | 16 ++++++++++++++--
 lib/compiler_rt/sincos.zig | 16 ++++++++++++++--
 lib/compiler_rt/tan.zig    | 16 ++++++++++++++--
 3 files changed, 42 insertions(+), 6 deletions(-)

diff --git a/lib/compiler_rt/sin.zig b/lib/compiler_rt/sin.zig
index 4ec2fa329e..e14779d27a 100644
--- a/lib/compiler_rt/sin.zig
+++ b/lib/compiler_rt/sin.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);
diff --git a/lib/compiler_rt/sincos.zig b/lib/compiler_rt/sincos.zig
index ec082ff436..72b90a51e7 100644
--- a/lib/compiler_rt/sincos.zig
+++ b/lib/compiler_rt/sincos.zig
@@ -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;
diff --git a/lib/compiler_rt/tan.zig b/lib/compiler_rt/tan.zig
index 7a87cfecad..af8d68eb44 100644
--- a/lib/compiler_rt/tan.zig
+++ b/lib/compiler_rt/tan.zig
@@ -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);