From 727e350f45ae71c703d1f666d011ff3fa983824a Mon Sep 17 00:00:00 2001
From: "Enzo \"raskyld\" Nocera" <enzo@nocera.eu>
Date: Sun, 6 Oct 2024 03:45:06 +0200
Subject: [PATCH 1/2] fix(int): avoid infinite recursion on left shift

Please, see this discussion for the full
context: https://rust-lang.zulipchat.com/#narrow/stream/131828-t-compiler/topic/.5Bwasm32.5D.20Infinite.20recursion.20.60compiler-builtins.60.20.60__multi3.60

Signed-off-by: Enzo "raskyld" Nocera <enzo@nocera.eu>

We determined that some recursion problems on SPARC and WASM were due to
infinite recusion. This was introduced at 9c6fcb56e8 ("Split Int into
Int and MinInt") when moving the implementation of `widen_hi` from
something on each `impl` block to a default on the trait. The reasoning
is not fully understood, but undoing this portion of the change seems to
resolve the issue.

[ add the above context - Trevor ]

Signed-off-by: Trevor Gross <tmgross@umich.edu>
---
 src/int/big.rs | 8 ++++++++
 src/int/mod.rs | 7 ++++---
 2 files changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/int/big.rs b/src/int/big.rs
index e565da89..0ef3caae 100644
--- a/src/int/big.rs
+++ b/src/int/big.rs
@@ -222,6 +222,10 @@ impl HInt for u128 {
     fn widen_mul(self, rhs: Self) -> Self::D {
         self.zero_widen_mul(rhs)
     }
+
+    fn widen_hi(self) -> Self::D {
+        self.widen() << <Self as MinInt>::BITS
+    }
 }
 
 impl HInt for i128 {
@@ -247,6 +251,10 @@ impl HInt for i128 {
     fn widen_mul(self, rhs: Self) -> Self::D {
         unimplemented!("signed i128 widening multiply is not used")
     }
+
+    fn widen_hi(self) -> Self::D {
+        self.widen() << <Self as MinInt>::BITS
+    }
 }
 
 impl DInt for u256 {
diff --git a/src/int/mod.rs b/src/int/mod.rs
index 5f56c6b6..c7ca45e7 100644
--- a/src/int/mod.rs
+++ b/src/int/mod.rs
@@ -319,9 +319,7 @@ pub(crate) trait HInt: Int {
     /// around problems with associated type bounds (such as `Int<Othersign: DInt>`) being unstable
     fn zero_widen(self) -> Self::D;
     /// Widens the integer to have double bit width and shifts the integer into the higher bits
-    fn widen_hi(self) -> Self::D {
-        self.widen() << <Self as MinInt>::BITS
-    }
+    fn widen_hi(self) -> Self::D;
     /// Widening multiplication with zero widening. This cannot overflow.
     fn zero_widen_mul(self, rhs: Self) -> Self::D;
     /// Widening multiplication. This cannot overflow.
@@ -364,6 +362,9 @@ macro_rules! impl_h_int {
                 fn widen_mul(self, rhs: Self) -> Self::D {
                     self.widen().wrapping_mul(rhs.widen())
                 }
+                fn widen_hi(self) -> Self::D {
+                    (self as $X) << <Self as MinInt>::BITS
+                }
             }
         )*
     };

From 87077a93dab529569fd7a8685148bb9851142aef Mon Sep 17 00:00:00 2001
From: Trevor Gross <tmgross@umich.edu>
Date: Sat, 5 Oct 2024 20:55:47 -0500
Subject: [PATCH 2/2] Add a note about avoiding default implemenations in some
 places

Link: https://github.com/rust-lang/compiler-builtins/pull/707
---
 src/int/mod.rs | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/src/int/mod.rs b/src/int/mod.rs
index c7ca45e7..e6f31c53 100644
--- a/src/int/mod.rs
+++ b/src/int/mod.rs
@@ -313,6 +313,10 @@ pub(crate) trait HInt: Int {
     /// Integer that is double the bit width of the integer this trait is implemented for
     type D: DInt<H = Self> + MinInt;
 
+    // NB: some of the below methods could have default implementations (e.g. `widen_hi`), but for
+    // unknown reasons this can cause infinite recursion when optimizations are disabled. See
+    // <https://github.com/rust-lang/compiler-builtins/pull/707> for context.
+
     /// Widens (using default extension) the integer to have double bit width
     fn widen(self) -> Self::D;
     /// Widens (zero extension only) the integer to have double bit width. This is needed to get