[clang] [llvm] [Clang][inlineasm] Add special support for "rm" output constraints (PR #92040)

[Nikita Popov via cfe-commits]

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@@ -0,0 +1,38 @@
+; RUN: llc -O0 -mtriple=x86_64-unknown-linux-gnu -stop-after=inline-asm-prepare < %s | FileCheck %s
+
+define void @test1(i32 %x) {
+; CHECK-LABEL: @test1
+; CHECK:         %asm_mem = alloca i32
+; CHECK-NEXT:    store i32 %x, ptr %asm_mem
+; CHECK-NEXT:    %0 = call i32 asm sideeffect "mov $1, $0", "=r,rm,~{dirflag},~{fpsr},~{flags}"(ptr %asm_mem)
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nikic wrote:

Sorry, I'm not super familiar with how the constraint handling works right now, and I'm a bit confused by what is going on here. As far as I can tell, the constraint string here stays the same, but we switch from an `i32` arg to a `ptr` arg. But how does the backend know about that? After all a pointer could also be for the `r` constraint, right? How are these distinguished?

https://github.com/llvm/llvm-project/pull/92040