Do not run mir opts for targets with convergent ops and add convergent attribute

compiler/rustc_codegen_llvm/src/allocator.rs

@@ -176,6 +176,15 @@ fn create_wrapper_function(
         None
     };
 
+    if tcx.sess.target.is_like_gpu {
+        // Conservatively apply convergent to all functions
+        attributes::apply_to_llfn(
+            llfn,
+            llvm::AttributePlace::Function,
+            &[llvm::AttributeKind::Convergent.create_attr(cx.llcx)],
+        );
+    }
+
     let llbb = unsafe { llvm::LLVMAppendBasicBlockInContext(cx.llcx, llfn, c"entry".as_ptr()) };
     let mut bx = SBuilder::build(&cx, llbb);
 

compiler/rustc_codegen_llvm/src/declare.rs

@@ -14,7 +14,7 @@
 use std::borrow::Borrow;
 
 use itertools::Itertools;
-use rustc_codegen_ssa::traits::TypeMembershipCodegenMethods;
+use rustc_codegen_ssa::traits::{MiscCodegenMethods, TypeMembershipCodegenMethods};
 use rustc_data_structures::fx::FxIndexSet;
 use rustc_middle::ty::{Instance, Ty};
 use rustc_sanitizers::{cfi, kcfi};
@@ -70,6 +70,11 @@ pub(crate) fn declare_raw_fn<'ll, 'tcx>(
 
     let mut attrs = SmallVec::<[_; 4]>::new();
 
+    if cx.sess().target.is_like_gpu {
+        // Conservatively apply convergent to all functions
+        attrs.push(llvm::AttributeKind::Convergent.create_attr(cx.llcx));
+    }
+
     if cx.tcx.sess.opts.cg.no_redzone.unwrap_or(cx.tcx.sess.target.disable_redzone) {
         attrs.push(llvm::AttributeKind::NoRedZone.create_attr(cx.llcx));
     }

compiler/rustc_codegen_llvm/src/llvm/ffi.rs

@@ -292,6 +292,7 @@ pub(crate) enum AttributeKind {
     CapturesNone = 46,
     SanitizeRealtimeNonblocking = 47,
     SanitizeRealtimeBlocking = 48,
+    Convergent = 49,
 }
 
 /// LLVMIntPredicate

compiler/rustc_llvm/llvm-wrapper/RustWrapper.cpp

@@ -328,6 +328,7 @@ enum class LLVMRustAttributeKind {
   CapturesNone = 46,
   SanitizeRealtimeNonblocking = 47,
   SanitizeRealtimeBlocking = 48,
+  Convergent = 49,
 };
 
 static Attribute::AttrKind fromRust(LLVMRustAttributeKind Kind) {
@@ -428,6 +429,8 @@ static Attribute::AttrKind fromRust(LLVMRustAttributeKind Kind) {
     return Attribute::SanitizeRealtime;
   case LLVMRustAttributeKind::SanitizeRealtimeBlocking:
     return Attribute::SanitizeRealtimeBlocking;
+  case LLVMRustAttributeKind::Convergent:
+    return Attribute::Convergent;
   }
   report_fatal_error("bad LLVMRustAttributeKind");
 }

compiler/rustc_session/src/session.rs

@@ -586,10 +586,16 @@ impl Session {
     }
 
     pub fn mir_opt_level(&self) -> usize {
-        self.opts
-            .unstable_opts
-            .mir_opt_level
-            .unwrap_or_else(|| if self.opts.optimize != OptLevel::No { 2 } else { 1 })
+        if self.target.is_like_gpu {
+            // Special care needs to be taken for convergent operations (i.e. not duplicating them).
+            // We do not want to handle these, so do not run any optimizations.
+            0
+        } else {
+            self.opts
+                .unstable_opts
+                .mir_opt_level
+                .unwrap_or_else(|| if self.opts.optimize != OptLevel::No { 2 } else { 1 })
+        }
     }
 
     /// Calculates the flavor of LTO to use for this compilation.

tests/codegen-llvm/gpu-convergent.rs

@@ -0,0 +1,28 @@
+// Checks that when compiling for GPU targets, the convergent attribute
+// is added to function declarations and definitions.
+
+//@ add-minicore
+//@ revisions: amdgpu nvptx
+//@ [amdgpu] compile-flags: --crate-type=rlib --target=amdgcn-amd-amdhsa -Ctarget-cpu=gfx900
+//@ [amdgpu] needs-llvm-components: amdgpu
+//@ [nvptx] compile-flags: --crate-type=rlib --target=nvptx64-nvidia-cuda
+//@ [nvptx] needs-llvm-components: nvptx
+#![feature(no_core, lang_items, abi_gpu_kernel)]
+#![no_core]
+
+extern crate minicore;
+use minicore::*;
+
+extern "C" {
+    fn ext();
+}
+
+// CHECK: define {{.*}}_kernel void @fun(i32{{.*}}) unnamed_addr #[[ATTR:[0-9]+]] {
+// CHECK: declare void @ext() unnamed_addr #[[ATTR]]
+// CHECK: attributes #[[ATTR]] = {{.*}} convergent
+#[no_mangle]
+pub extern "gpu-kernel" fn fun(_: i32) {
+    unsafe {
+        ext();
+    }
+}