Auto merge of #120718 - saethlin:reasonable-fast-math, r=nnethercote

Add "algebraic" fast-math intrinsics, based on fast-math ops that cannot return poison

Setting all of LLVM's fast-math flags makes our fast-math intrinsics very dangerous, because some inputs are UB. This set of flags permits common algebraic transformations, but according to the [LangRef](https://llvm.org/docs/LangRef.html#fastmath), only the flags `nnan` (no nans) and `ninf` (no infs) can produce poison.

And this uses the algebraic float ops to fix https://github.com/rust-lang/rust/issues/120720

cc `@orlp`

2 files changed, 37 insertions, 0 deletions

diff --git a/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs b/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs
index e4633acd817..82488829b6e 100644
--- a/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/intrinsic.rs
@@ -250,6 +250,38 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
                     }
                 }
             }
+            sym::fadd_algebraic
+            | sym::fsub_algebraic
+            | sym::fmul_algebraic
+            | sym::fdiv_algebraic
+            | sym::frem_algebraic => match float_type_width(arg_tys[0]) {
+                Some(_width) => match name {
+                    sym::fadd_algebraic => {
+                        bx.fadd_algebraic(args[0].immediate(), args[1].immediate())
+                    }
+                    sym::fsub_algebraic => {
+                        bx.fsub_algebraic(args[0].immediate(), args[1].immediate())
+                    }
+                    sym::fmul_algebraic => {
+                        bx.fmul_algebraic(args[0].immediate(), args[1].immediate())
+                    }
+                    sym::fdiv_algebraic => {
+                        bx.fdiv_algebraic(args[0].immediate(), args[1].immediate())
+                    }
+                    sym::frem_algebraic => {
+                        bx.frem_algebraic(args[0].immediate(), args[1].immediate())
+                    }
+                    _ => bug!(),
+                },
+                None => {
+                    bx.tcx().dcx().emit_err(InvalidMonomorphization::BasicFloatType {
+                        span,
+                        name,
+                        ty: arg_tys[0],
+                    });
+                    return Ok(());
+                }
+            },
 
             sym::float_to_int_unchecked => {
                 if float_type_width(arg_tys[0]).is_none() {
diff --git a/compiler/rustc_codegen_ssa/src/traits/builder.rs b/compiler/rustc_codegen_ssa/src/traits/builder.rs
index 1c5c78e6ca2..86d3d1260c3 100644
--- a/compiler/rustc_codegen_ssa/src/traits/builder.rs
+++ b/compiler/rustc_codegen_ssa/src/traits/builder.rs
@@ -86,22 +86,27 @@ pub trait BuilderMethods<'a, 'tcx>:
     fn add(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn fadd(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn fadd_fast(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn fadd_algebraic(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn sub(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn fsub(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn fsub_fast(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn fsub_algebraic(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn mul(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn fmul(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn fmul_fast(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn fmul_algebraic(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn udiv(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn exactudiv(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn sdiv(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn exactsdiv(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn fdiv(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn fdiv_fast(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn fdiv_algebraic(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn urem(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn srem(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn frem(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn frem_fast(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
+    fn frem_algebraic(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn shl(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn lshr(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;
     fn ashr(&mut self, lhs: Self::Value, rhs: Self::Value) -> Self::Value;