undo CL 19810043 / 352f3b7c9664

The CL causes misc/cgo/test to fail randomly.
I suspect that the problem is the use of a division instruction
in usleep, which can be called while trying to acquire an m
and therefore cannot store the denominator in m.
The solution to that would be to rewrite the code to use a
magic multiply instead of a divide, but now we're getting
pretty far off the original code.

Go back to the original in preparation for a different,
less efficient but simpler fix.

««« original CL description
cmd/5l, runtime: make ARM integer division profiler-friendly

The implementation of division constructed non-standard
stack frames that could not be handled by the traceback
routines.

CL 13239052 left the frames non-standard but fixed them
for the specific case of a divide-by-zero panic.
A profiling signal can arrive at any time, so that fix
is not sufficient.

Change the division to store the extra argument in the M struct
instead of in a new stack slot. That keeps the frames bog standard
at all times.

Also fix a related bug in the traceback code: when starting
a traceback, the LR register should be ignored if the current
function has already allocated its stack frame and saved the
original LR on the stack. The stack copy should be used, as the
LR register may have been modified.

Combined, these make the torture test from issue 6681 pass.

Fixes #6681.

R=golang-dev, r, josharian
CC=golang-dev
https://golang.org/cl/19810043
»»»

TBR=r
CC=golang-dev
https://golang.org/cl/20350043

diff --git a/src/cmd/5l/noop.c b/src/cmd/5l/noop.c
index 70cec1f9ce142658d50fca2df87ee51551921703..fb70599b514951ec60533faea0f6cf9339c59fa4 100644 (file)
--- a/src/cmd/5l/noop.c
+++ b/src/cmd/5l/noop.c
@@ -60,7 +60,7 @@ linkcase(Prog *casep)
 void
 noops(void)
 {
-       Prog *p, *q, *q1, *q2, orig;
+       Prog *p, *q, *q1, *q2;
        int o;
        Sym *tlsfallback, *gmsym;
 
@@ -401,27 +401,26 @@ noops(void)
                                        break;
                                if(p->to.type != D_REG)
                                        break;
-
-                               orig = *p;
+                               q1 = p;
        
-                               /* MOV a,4(M) */
+                               /* MOV a,4(SP) */
+                               p = appendp(p);
                                p->as = AMOVW;
-                               p->line = orig.line;
+                               p->line = q1->line;
                                p->from.type = D_REG;
-                               p->from.reg = orig.from.reg;
-                               p->reg = NREG;
+                               p->from.reg = q1->from.reg;
                                p->to.type = D_OREG;
-                               p->to.reg = REGM;
+                               p->to.reg = REGSP;
                                p->to.offset = 4;
        
                                /* MOV b,REGTMP */
                                p = appendp(p);
                                p->as = AMOVW;
-                               p->line = orig.line;
+                               p->line = q1->line;
                                p->from.type = D_REG;
-                               p->from.reg = orig.reg;
-                               if(orig.reg == NREG)
-                                       p->from.reg = orig.to.reg;
+                               p->from.reg = q1->reg;
+                               if(q1->reg == NREG)
+                                       p->from.reg = q1->to.reg;
                                p->to.type = D_REG;
                                p->to.reg = REGTMP;
                                p->to.offset = 0;
@@ -429,7 +428,7 @@ noops(void)
                                /* CALL appropriate */
                                p = appendp(p);
                                p->as = ABL;
-                               p->line = orig.line;
+                               p->line = q1->line;
                                p->to.type = D_BRANCH;
                                p->cond = p;
                                switch(o) {
@@ -454,12 +453,34 @@ noops(void)
                                /* MOV REGTMP, b */
                                p = appendp(p);
                                p->as = AMOVW;
-                               p->line = orig.line;
+                               p->line = q1->line;
                                p->from.type = D_REG;
                                p->from.reg = REGTMP;
                                p->from.offset = 0;
                                p->to.type = D_REG;
-                               p->to.reg = orig.to.reg;
+                               p->to.reg = q1->to.reg;
+       
+                               /* ADD $8,SP */
+                               p = appendp(p);
+                               p->as = AADD;
+                               p->line = q1->line;
+                               p->from.type = D_CONST;
+                               p->from.reg = NREG;
+                               p->from.offset = 8;
+                               p->reg = NREG;
+                               p->to.type = D_REG;
+                               p->to.reg = REGSP;
+                               p->spadj = -8;
+       
+                               /* SUB $8,SP */
+                               q1->as = ASUB;
+                               q1->from.type = D_CONST;
+                               q1->from.offset = 8;
+                               q1->from.reg = NREG;
+                               q1->reg = NREG;
+                               q1->to.type = D_REG;
+                               q1->to.reg = REGSP;
+                               q1->spadj = 8;
        
                                break;
                        case AMOVW:

diff --git a/src/pkg/runtime/pprof/pprof_test.go b/src/pkg/runtime/pprof/pprof_test.go
index eb76b93c44cfe2e584f28d49eec470f7f550fd4f..f1fc5faec689d050fc7124a8f0621801f36af4f8 100644 (file)
--- a/src/pkg/runtime/pprof/pprof_test.go
+++ b/src/pkg/runtime/pprof/pprof_test.go
@@ -8,7 +8,6 @@ import (
        "bytes"
        "fmt"
        "hash/crc32"
-       "math/big"
        "os/exec"
        "regexp"
        "runtime"
@@ -124,10 +123,6 @@ func testCPUProfile(t *testing.T, need []string, f func()) {
                }
        })
 
-       if len(need) == 0 {
-               return
-       }
-
        var total uintptr
        for i, name := range need {
                total += have[i]
@@ -242,26 +237,6 @@ func TestGoroutineSwitch(t *testing.T) {
        }
 }
 
-// Test that profiling of division operations is okay, especially on ARM. See issue 6681.
-func TestMathBigDivide(t *testing.T) {
-       testCPUProfile(t, nil, func() {
-               t := time.After(5 * time.Second)
-               pi := new(big.Int)
-               for {
-                       for i := 0; i < 100; i++ {
-                               n := big.NewInt(2646693125139304345)
-                               d := big.NewInt(842468587426513207)
-                               pi.Div(n, d)
-                       }
-                       select {
-                       case <-t:
-                               return
-                       default:
-                       }
-               }
-       })
-}
-
 // Operating systems that are expected to fail the tests. See issue 6047.
 var badOS = map[string]bool{
        "darwin":  true,

diff --git a/src/pkg/runtime/runtime.h b/src/pkg/runtime/runtime.h
index 02c7041ba7f1d5074d0148cd439a1e7350a223e6..f7c2adb12192fc5c6199b9d24e919274910a3c91 100644 (file)
--- a/src/pkg/runtime/runtime.h
+++ b/src/pkg/runtime/runtime.h
@@ -290,7 +290,6 @@ struct      G
 struct M
 {
        G*      g0;             // goroutine with scheduling stack
-       uint32  divmod;         // div/mod denominator on arm
        void*   moreargp;       // argument pointer for more stack
        Gobuf   morebuf;        // gobuf arg to morestack
 

diff --git a/src/pkg/runtime/traceback_arm.c b/src/pkg/runtime/traceback_arm.c
index 7c21dc7981693360127fc762e203f28505e4fb6c..02586f036bba6dad9d60a27fab63b8b14ea5a510 100644 (file)
--- a/src/pkg/runtime/traceback_arm.c
+++ b/src/pkg/runtime/traceback_arm.c
@@ -84,7 +84,7 @@ runtime·gentraceback(uintptr pc0, uintptr sp0, uintptr lr0, G *gp, int32 skip,
                        frame.lr = 0;
                        flr = nil;
                } else {
-                       if((n == 0 && frame.fp > frame.sp) || frame.lr == 0)
+                       if(frame.lr == 0)
                                frame.lr = *(uintptr*)frame.sp;
                        flr = runtime·findfunc(frame.lr);
                        if(flr == nil) {

diff --git a/src/pkg/runtime/vlop_arm.s b/src/pkg/runtime/vlop_arm.s
index 76c0d5dabb0d83a2f1916267e3585711cbd50f36..d7c566afb87acbdc2b45221bf9cc0e99d7e77b20 100644 (file)
--- a/src/pkg/runtime/vlop_arm.s
+++ b/src/pkg/runtime/vlop_arm.s
@@ -105,7 +105,7 @@ s = 2 // three temporary variables
 M = 3
 a = 11
 // Be careful: R(a) == R11 will be used by the linker for synthesized instructions.
-TEXT udiv<>(SB),NOSPLIT,$-4-0
+TEXT udiv<>(SB),NOSPLIT,$-4
        CLZ     R(q), R(s) // find normalizing shift
        MOVW.S  R(q)<<R(s), R(a)
        MOVW    $fast_udiv_tab<>-64(SB), R(M)
@@ -165,8 +165,22 @@ udiv_by_0_or_1:
        RET
 
 udiv_by_0:
-       MOVW    $runtime·panicdivide(SB),R11
-       B       (R11)
+       // The ARM toolchain expects it can emit references to DIV and MOD
+       // instructions. The linker rewrites each pseudo-instruction into
+       // a sequence that pushes two values onto the stack and then calls
+       // _divu, _modu, _div, or _mod (below), all of which have a 16-byte
+       // frame plus the saved LR. The traceback routine knows the expanded
+       // stack frame size at the pseudo-instruction call site, but it
+       // doesn't know that the frame has a non-standard layout. In particular,
+       // it expects to find a saved LR in the bottom word of the frame.
+       // Unwind the stack back to the pseudo-instruction call site, copy the
+       // saved LR where the traceback routine will look for it, and make it
+       // appear that panicdivide was called from that PC.
+       MOVW    0(R13), LR
+       ADD     $20, R13
+       MOVW    8(R13), R1 // actual saved LR
+       MOVW    R1, 0(R13) // expected here for traceback
+       B       runtime·panicdivide(SB)
 
 TEXT fast_udiv_tab<>(SB),NOSPLIT,$-4
        // var tab [64]byte
@@ -193,16 +207,14 @@ TEXT fast_udiv_tab<>(SB),NOSPLIT,$-4
 // expects the result in R(TMP)
 TMP = 11
 
-TEXT _divu(SB), NOSPLIT, $16-0
+TEXT _divu(SB), NOSPLIT, $16
        MOVW    R(q), 4(R13)
        MOVW    R(r), 8(R13)
        MOVW    R(s), 12(R13)
        MOVW    R(M), 16(R13)
 
        MOVW    R(TMP), R(r)            /* numerator */
-       MOVW    m_divmod(m), R(q)       /* denominator */
-       MOVW    $0, R(s)
-       MOVW    R(s), m_divmod(m)
+       MOVW    0(FP), R(q)             /* denominator */
        BL      udiv<>(SB)
        MOVW    R(q), R(TMP)
        MOVW    4(R13), R(q)
@@ -211,16 +223,14 @@ TEXT _divu(SB), NOSPLIT, $16-0
        MOVW    16(R13), R(M)
        RET
 
-TEXT _modu(SB), NOSPLIT, $16-0
+TEXT _modu(SB), NOSPLIT, $16
        MOVW    R(q), 4(R13)
        MOVW    R(r), 8(R13)
        MOVW    R(s), 12(R13)
        MOVW    R(M), 16(R13)
 
        MOVW    R(TMP), R(r)            /* numerator */
-       MOVW    m_divmod(m), R(q)       /* denominator */
-       MOVW    $0, R(s)
-       MOVW    R(s), m_divmod(m)
+       MOVW    0(FP), R(q)             /* denominator */
        BL      udiv<>(SB)
        MOVW    R(r), R(TMP)
        MOVW    4(R13), R(q)
@@ -229,15 +239,13 @@ TEXT _modu(SB), NOSPLIT, $16-0
        MOVW    16(R13), R(M)
        RET
 
-TEXT _div(SB),NOSPLIT,$16-0
+TEXT _div(SB),NOSPLIT,$16
        MOVW    R(q), 4(R13)
        MOVW    R(r), 8(R13)
        MOVW    R(s), 12(R13)
        MOVW    R(M), 16(R13)
        MOVW    R(TMP), R(r)            /* numerator */
-       MOVW    m_divmod(m), R(q)               /* denominator */
-       MOVW    $0, R(s)
-       MOVW    R(s), m_divmod(m)
+       MOVW    0(FP), R(q)             /* denominator */
        CMP     $0, R(r)
        BGE     d1
        RSB     $0, R(r), R(r)
@@ -257,15 +265,13 @@ d2:
        RSB             $0, R(q), R(TMP)
        B       out
 
-TEXT _mod(SB),NOSPLIT,$16-0
+TEXT _mod(SB),NOSPLIT,$16
        MOVW    R(q), 4(R13)
        MOVW    R(r), 8(R13)
        MOVW    R(s), 12(R13)
        MOVW    R(M), 16(R13)
        MOVW    R(TMP), R(r)            /* numerator */
-       MOVW    m_divmod(m), R(q)               /* denominator */
-       MOVW    $0, R(s)
-       MOVW    R(s), m_divmod(m)
+       MOVW    0(FP), R(q)             /* denominator */
        CMP     $0, R(q)
        RSB.LT  $0, R(q), R(q)
        CMP     $0, R(r)