rust/SOURCES/0001-Disable-jump-threading-of-float-equality.patch

From 49166c7dd925244f631277b4aa9ae4233f300884 Mon Sep 17 00:00:00 2001
From: Nilstrieb <48135649+Nilstrieb@users.noreply.github.com>
Date: Sat, 27 Jul 2024 15:08:11 +0200
Subject: [PATCH] Disable jump threading of float equality

Jump threading stores values as `u128` (`ScalarInt`) and does its
comparisons for equality as integer comparisons.
This works great for integers. Sadly, not everything is an integer.

Floats famously have wonky equality semantcs, with `NaN!=NaN` and
`0.0 == -0.0`. This does not match our beautiful integer bitpattern
equality and therefore causes things to go horribly wrong.

While jump threading could be extended to support floats by remembering
that they're floats in the value state and handling them properly,
it's signficantly easier to just disable it for now.

(cherry picked from commit eca0a7e72346ba123ace318a0f9c28c57d990aeb)
---
 .../rustc_mir_transform/src/jump_threading.rs |  7 +++
 ...ding.floats.JumpThreading.panic-abort.diff | 59 +++++++++++++++++++
 ...ing.floats.JumpThreading.panic-unwind.diff | 59 +++++++++++++++++++
 tests/mir-opt/jump_threading.rs               | 12 ++++
 4 files changed, 137 insertions(+)
 create mode 100644 tests/mir-opt/jump_threading.floats.JumpThreading.panic-abort.diff
 create mode 100644 tests/mir-opt/jump_threading.floats.JumpThreading.panic-unwind.diff

diff --git a/compiler/rustc_mir_transform/src/jump_threading.rs b/compiler/rustc_mir_transform/src/jump_threading.rs
index a458297210db..e2d2864ad2a0 100644
--- a/compiler/rustc_mir_transform/src/jump_threading.rs
+++ b/compiler/rustc_mir_transform/src/jump_threading.rs
@@ -493,6 +493,13 @@ fn process_assign(
                     BinOp::Ne => ScalarInt::FALSE,
                     _ => return None,
                 };
+                if value.const_.ty().is_floating_point() {
+                    // Floating point equality does not follow bit-patterns.
+                    // -0.0 and NaN both have special rules for equality,
+                    // and therefore we cannot use integer comparisons for them.
+                    // Avoid handling them, though this could be extended in the future.
+                    return None;
+                }
                 let value = value.const_.normalize(self.tcx, self.param_env).try_to_scalar_int()?;
                 let conds = conditions.map(self.arena, |c| Condition {
                     value,
diff --git a/tests/mir-opt/jump_threading.floats.JumpThreading.panic-abort.diff b/tests/mir-opt/jump_threading.floats.JumpThreading.panic-abort.diff
new file mode 100644
index 000000000000..6ca37e96d297
--- /dev/null
+++ b/tests/mir-opt/jump_threading.floats.JumpThreading.panic-abort.diff
@@ -0,0 +1,59 @@
+- // MIR for `floats` before JumpThreading
++ // MIR for `floats` after JumpThreading
+  
+  fn floats() -> u32 {
+      let mut _0: u32;
+      let _1: f64;
+      let mut _2: bool;
+      let mut _3: bool;
+      let mut _4: f64;
+      scope 1 {
+          debug x => _1;
+      }
+  
+      bb0: {
+          StorageLive(_1);
+          StorageLive(_2);
+          _2 = const true;
+-         switchInt(move _2) -> [0: bb2, otherwise: bb1];
++         goto -> bb1;
+      }
+  
+      bb1: {
+          _1 = const -0f64;
+          goto -> bb3;
+      }
+  
+      bb2: {
+          _1 = const 1f64;
+          goto -> bb3;
+      }
+  
+      bb3: {
+          StorageDead(_2);
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = _1;
+          _3 = Eq(move _4, const 0f64);
+          switchInt(move _3) -> [0: bb5, otherwise: bb4];
+      }
+  
+      bb4: {
+          StorageDead(_4);
+          _0 = const 0_u32;
+          goto -> bb6;
+      }
+  
+      bb5: {
+          StorageDead(_4);
+          _0 = const 1_u32;
+          goto -> bb6;
+      }
+  
+      bb6: {
+          StorageDead(_3);
+          StorageDead(_1);
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/jump_threading.floats.JumpThreading.panic-unwind.diff b/tests/mir-opt/jump_threading.floats.JumpThreading.panic-unwind.diff
new file mode 100644
index 000000000000..6ca37e96d297
--- /dev/null
+++ b/tests/mir-opt/jump_threading.floats.JumpThreading.panic-unwind.diff
@@ -0,0 +1,59 @@
+- // MIR for `floats` before JumpThreading
++ // MIR for `floats` after JumpThreading
+  
+  fn floats() -> u32 {
+      let mut _0: u32;
+      let _1: f64;
+      let mut _2: bool;
+      let mut _3: bool;
+      let mut _4: f64;
+      scope 1 {
+          debug x => _1;
+      }
+  
+      bb0: {
+          StorageLive(_1);
+          StorageLive(_2);
+          _2 = const true;
+-         switchInt(move _2) -> [0: bb2, otherwise: bb1];
++         goto -> bb1;
+      }
+  
+      bb1: {
+          _1 = const -0f64;
+          goto -> bb3;
+      }
+  
+      bb2: {
+          _1 = const 1f64;
+          goto -> bb3;
+      }
+  
+      bb3: {
+          StorageDead(_2);
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = _1;
+          _3 = Eq(move _4, const 0f64);
+          switchInt(move _3) -> [0: bb5, otherwise: bb4];
+      }
+  
+      bb4: {
+          StorageDead(_4);
+          _0 = const 0_u32;
+          goto -> bb6;
+      }
+  
+      bb5: {
+          StorageDead(_4);
+          _0 = const 1_u32;
+          goto -> bb6;
+      }
+  
+      bb6: {
+          StorageDead(_3);
+          StorageDead(_1);
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/jump_threading.rs b/tests/mir-opt/jump_threading.rs
index 57f4e4a2654f..3e7e8995f1a3 100644
--- a/tests/mir-opt/jump_threading.rs
+++ b/tests/mir-opt/jump_threading.rs
@@ -514,6 +514,16 @@ fn assume(a: u8, b: bool) -> u8 {
     )
 }
 
+fn floats() -> u32 {
+    // CHECK-LABEL: fn floats(
+    // CHECK: switchInt(
+
+    // Test for issue #128243, where float equality was assumed to be bitwise.
+    // When adding float support, it must be ensured that this continues working properly.
+    let x = if true { -0.0 } else { 1.0 };
+    if x == 0.0 { 0 } else { 1 }
+}
+
 fn main() {
     // CHECK-LABEL: fn main(
     too_complex(Ok(0));
@@ -528,6 +538,7 @@ fn main() {
     disappearing_bb(7);
     aggregate(7);
     assume(7, false);
+    floats();
 }
 
 // EMIT_MIR jump_threading.too_complex.JumpThreading.diff
@@ -542,3 +553,4 @@ fn main() {
 // EMIT_MIR jump_threading.disappearing_bb.JumpThreading.diff
 // EMIT_MIR jump_threading.aggregate.JumpThreading.diff
 // EMIT_MIR jump_threading.assume.JumpThreading.diff
+// EMIT_MIR jump_threading.floats.JumpThreading.diff
-- 
2.46.0