[dev.ssa] cmd/compile: fix stackalloc handling of zero-aligned variables

Prior to this fix, a zero-aligned variable such as a flags
variable would reset n to 0.

While we're here, log the stack layout so that debugging
and reading the generated assembly is easier.

Change-Id: I18ef83ea95b6ea877c83f2e595e14c48c9ad7d84
Reviewed-on: https://go-review.googlesource.com/12439
Reviewed-by: Keith Randall <khr@golang.org>

diff --git a/src/cmd/compile/internal/ssa/stackalloc.go b/src/cmd/compile/internal/ssa/stackalloc.go
index 5f18acabfd310089888005eda0fce19fc3942d02..2d639bf59425bf6f208144d9d81c1f3f45aa998c 100644 (file)
--- a/src/cmd/compile/internal/ssa/stackalloc.go
+++ b/src/cmd/compile/internal/ssa/stackalloc.go
@@ -20,6 +20,7 @@ func stackalloc(f *Func) {
                        n = v.AuxInt
                }
        }
+       f.Logf("stackalloc: 0-%d for callee arguments/returns\n", n)
 
        // TODO: group variables by ptr/nonptr, size, etc.  Emit ptr vars last
        // so stackmap is smaller.
@@ -36,6 +37,7 @@ func stackalloc(f *Func) {
                                continue
                        }
                        n = align(n, v.Type.Alignment())
+                       f.Logf("stackalloc: %d-%d for %v\n", n, n+v.Type.Size(), v)
                        loc := &LocalSlot{n}
                        n += v.Type.Size()
                        home = setloc(home, v, loc)
@@ -62,6 +64,7 @@ func stackalloc(f *Func) {
                                continue
                        }
                        n = align(n, v.Type.Alignment())
+                       f.Logf("stackalloc: %d-%d for %v\n", n, n+v.Type.Size(), v)
                        loc := &LocalSlot{n}
                        n += v.Type.Size()
                        home = setloc(home, v, loc)
@@ -77,12 +80,14 @@ func stackalloc(f *Func) {
                        }
                        t := s.Typ
                        n = align(n, t.Alignment())
+                       f.Logf("stackalloc: %d-%d for auto %v\n", n, n+t.Size(), v)
                        s.Offset = n
                        n += t.Size()
                }
        }
 
        n = align(n, f.Config.PtrSize)
+       f.Logf("stackalloc: %d-%d for return address\n", n, n+f.Config.ptrSize)
        n += f.Config.PtrSize // space for return address.  TODO: arch-dependent
        f.RegAlloc = home
        f.FrameSize = n
@@ -92,5 +97,8 @@ func stackalloc(f *Func) {
 
 // align increases n to the next multiple of a.  a must be a power of 2.
 func align(n int64, a int64) int64 {
+       if a == 0 {
+               return n
+       }
        return (n + a - 1) &^ (a - 1)
 }