Commits on Sep 17, 2024

1. Reorder stack spills so that constants come later. …

   Currently constants are "pulled forward" and have their stack spills emitted
   first. This confuses LLVM as to where to place breakpoints at function
   entry, and results in argument values being wrong in the debugger. It's
   straightforward to avoid emitting the stack spills for constants until
   arguments/etc have been introduced in debug_introduce_locals, so do that.
   
   Example LLVM IR (irrelevant IR elided):
   Before:
   
   define internal void @_ZN11rust_1289457binding17h2c78f956ba4bd2c3E(i64 %a, i64 %b, double %c) unnamed_addr #0 !dbg !178 {
   start:
     %c.dbg.spill = alloca [8 x i8], align 8
     %b.dbg.spill = alloca [8 x i8], align 8
     %a.dbg.spill = alloca [8 x i8], align 8
     %x.dbg.spill = alloca [4 x i8], align 4
     store i32 0, ptr %x.dbg.spill, align 4, !dbg !192            ; LLVM places breakpoint here.
       #dbg_declare(ptr %x.dbg.spill, !190, !DIExpression(), !192)
     store i64 %a, ptr %a.dbg.spill, align 8
       #dbg_declare(ptr %a.dbg.spill, !187, !DIExpression(), !193)
     store i64 %b, ptr %b.dbg.spill, align 8
       #dbg_declare(ptr %b.dbg.spill, !188, !DIExpression(), !194)
     store double %c, ptr %c.dbg.spill, align 8
       #dbg_declare(ptr %c.dbg.spill, !189, !DIExpression(), !195)
     ret void, !dbg !196
   }
   
   After:
   define internal void @_ZN11rust_1289457binding17h2c78f956ba4bd2c3E(i64 %a, i64 %b, double %c) unnamed_addr #0 !dbg !178 {
   start:
     %x.dbg.spill = alloca [4 x i8], align 4
     %c.dbg.spill = alloca [8 x i8], align 8
     %b.dbg.spill = alloca [8 x i8], align 8
     %a.dbg.spill = alloca [8 x i8], align 8
     store i64 %a, ptr %a.dbg.spill, align 8
       #dbg_declare(ptr %a.dbg.spill, !187, !DIExpression(), !192)
     store i64 %b, ptr %b.dbg.spill, align 8
       #dbg_declare(ptr %b.dbg.spill, !188, !DIExpression(), !193)
     store double %c, ptr %c.dbg.spill, align 8
       #dbg_declare(ptr %c.dbg.spill, !189, !DIExpression(), !194)
     store i32 0, ptr %x.dbg.spill, align 4, !dbg !195            ; LLVM places breakpoint here.
       #dbg_declare(ptr %x.dbg.spill, !190, !DIExpression(), !195)
     ret void, !dbg !196
   }
   
   Note in particular the position of the "LLVM places breakpoint here" comment
   relative to the stack spills for the function arguments. LLVM assumes that
   the first instruction with with a debug location is the end of the prologue.
   As LLVM does not currently offer front ends any direct control over the
   placement of the prologue end reordering the IR is the only mechanism available
   to fix argument values at function entry in the presence of MIR optimizations
   like SingleUseConsts. Fixes rust-lang#128945

   

   khuey committed Sep 17, 2024
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