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`

compiler/rustc_codegen_llvm/src/builder.rs

@@ -340,6 +340,46 @@ impl<'a, 'll, 'tcx> BuilderMethods<'a, 'tcx> for Builder<'a, 'll, 'tcx> {
         }
     }
 
+    fn fadd_algebraic(&mut self, lhs: &'ll Value, rhs: &'ll Value) -> &'ll Value {
+        unsafe {
+            let instr = llvm::LLVMBuildFAdd(self.llbuilder, lhs, rhs, UNNAMED);
+            llvm::LLVMRustSetAlgebraicMath(instr);
+            instr
+        }
+    }
+
+    fn fsub_algebraic(&mut self, lhs: &'ll Value, rhs: &'ll Value) -> &'ll Value {
+        unsafe {
+            let instr = llvm::LLVMBuildFSub(self.llbuilder, lhs, rhs, UNNAMED);
+            llvm::LLVMRustSetAlgebraicMath(instr);
+            instr
+        }
+    }
+
+    fn fmul_algebraic(&mut self, lhs: &'ll Value, rhs: &'ll Value) -> &'ll Value {
+        unsafe {
+            let instr = llvm::LLVMBuildFMul(self.llbuilder, lhs, rhs, UNNAMED);
+            llvm::LLVMRustSetAlgebraicMath(instr);
+            instr
+        }
+    }
+
+    fn fdiv_algebraic(&mut self, lhs: &'ll Value, rhs: &'ll Value) -> &'ll Value {
+        unsafe {
+            let instr = llvm::LLVMBuildFDiv(self.llbuilder, lhs, rhs, UNNAMED);
+            llvm::LLVMRustSetAlgebraicMath(instr);
+            instr
+        }
+    }
+
+    fn frem_algebraic(&mut self, lhs: &'ll Value, rhs: &'ll Value) -> &'ll Value {
+        unsafe {
+            let instr = llvm::LLVMBuildFRem(self.llbuilder, lhs, rhs, UNNAMED);
+            llvm::LLVMRustSetAlgebraicMath(instr);
+            instr
+        }
+    }
+
     fn checked_binop(
         &mut self,
         oop: OverflowOp,
@@ -1327,17 +1367,17 @@ impl<'a, 'll, 'tcx> Builder<'a, 'll, 'tcx> {
     pub fn vector_reduce_fmul(&mut self, acc: &'ll Value, src: &'ll Value) -> &'ll Value {
         unsafe { llvm::LLVMRustBuildVectorReduceFMul(self.llbuilder, acc, src) }
     }
-    pub fn vector_reduce_fadd_fast(&mut self, acc: &'ll Value, src: &'ll Value) -> &'ll Value {
+    pub fn vector_reduce_fadd_algebraic(&mut self, acc: &'ll Value, src: &'ll Value) -> &'ll Value {
         unsafe {
             let instr = llvm::LLVMRustBuildVectorReduceFAdd(self.llbuilder, acc, src);
-            llvm::LLVMRustSetFastMath(instr);
+            llvm::LLVMRustSetAlgebraicMath(instr);
             instr
         }
     }
-    pub fn vector_reduce_fmul_fast(&mut self, acc: &'ll Value, src: &'ll Value) -> &'ll Value {
+    pub fn vector_reduce_fmul_algebraic(&mut self, acc: &'ll Value, src: &'ll Value) -> &'ll Value {
         unsafe {
             let instr = llvm::LLVMRustBuildVectorReduceFMul(self.llbuilder, acc, src);
-            llvm::LLVMRustSetFastMath(instr);
+            llvm::LLVMRustSetAlgebraicMath(instr);
             instr
         }
     }

compiler/rustc_codegen_llvm/src/intrinsic.rs

@@ -1880,14 +1880,14 @@ fn generic_simd_intrinsic<'ll, 'tcx>(
     arith_red!(simd_reduce_mul_ordered: vector_reduce_mul, vector_reduce_fmul, true, mul, 1.0);
     arith_red!(
         simd_reduce_add_unordered: vector_reduce_add,
-        vector_reduce_fadd_fast,
+        vector_reduce_fadd_algebraic,
         false,
         add,
         0.0
     );
     arith_red!(
         simd_reduce_mul_unordered: vector_reduce_mul,
-        vector_reduce_fmul_fast,
+        vector_reduce_fmul_algebraic,
         false,
         mul,
         1.0

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -1618,6 +1618,7 @@ extern "C" {
     ) -> &'a Value;
 
     pub fn LLVMRustSetFastMath(Instr: &Value);
+    pub fn LLVMRustSetAlgebraicMath(Instr: &Value);
 
     // Miscellaneous instructions
     pub fn LLVMRustGetInstrProfIncrementIntrinsic(M: &Module) -> &Value;