runtime: change top-most return PC from goexit to goexit+PCQuantum
If you get a stack of PCs from Callers, it would be expected
that every PC is immediately after a call instruction, so to find
the line of the call, you look up the line for PC-1.
CL 163550043 now explicitly documents that.
The most common exception to this is the top-most return PC
on the stack, which is the entry address of the runtime.goexit
function. Subtracting 1 from that PC will end up in a different
function entirely.
To remove this special case, make the top-most return PC
goexit+PCQuantum and then implement goexit in assembly
so that the first instruction can be skipped.
Fixes #7690.
LGTM=r
R=r
CC=golang-codereviews
https://golang.org/cl/170720043
diff --git a/src/runtime/asm_386.s b/src/runtime/asm_386.s
index 0d46a9e..b4b81d7 100644
--- a/src/runtime/asm_386.s
+++ b/src/runtime/asm_386.s
@@ -2284,3 +2284,9 @@
MOVL m_curg(AX), AX
MOVL (g_stack+stack_hi)(AX), AX
RET
+
+// The top-most function running on a goroutine
+// returns to goexit+PCQuantum.
+TEXT runtime·goexit(SB),NOSPLIT,$0-0
+ BYTE $0x90 // NOP
+ CALL runtime·goexit1(SB) // does not return
