[llvm-bugs] [Bug 45865] New: Passing values to inline assembly via high-byte registers can cause unencodable instructions

[via llvm-bugs]

https://bugs.llvm.org/show_bug.cgi?id=45865

            Bug ID: 45865
           Summary: Passing values to inline assembly via high-byte
                    registers can cause unencodable instructions
           Product: new-bugs
           Version: 10.0
          Hardware: PC
                OS: Linux
            Status: NEW
          Severity: normal
          Priority: P
         Component: new bugs
          Assignee: unassignedbugs at nondot.org
          Reporter: josephcsible at gmail.com
                CC: htmldeveloper at gmail.com, llvm-bugs at lists.llvm.org

Created attachment 23467
  --> https://bugs.llvm.org/attachment.cgi?id=23467&action=edit
Result of clang -O -emit-llvm -c -o foo.bc foo.c && bugpoint -llc-safe foo.bc

Consider this minimal C function:

void f(char x) {
    register char y asm("ah") = x;
    asm volatile("" :: "r"(y));
}

When I compile it with any optimization level except -O0, the code generator
fails with "Cannot encode high byte register in REX-prefixed instruction".

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