add -K flag to check stack underflow

R=ken
DELTA=38  (36 added, 0 deleted, 2 changed)
OCL=15482
CL=15491

diff --git a/src/cmd/6l/pass.c b/src/cmd/6l/pass.c
index fed9495c1773a7c1333b55eba947bbffb065506c..39394026dc3e1071ae425e28dcbcbac222874f0b 100644 (file)
--- a/src/cmd/6l/pass.c
+++ b/src/cmd/6l/pass.c
@@ -565,7 +565,7 @@ brloop(Prog *p)
 void
 dostkoff(void)
 {
-       Prog *p, *q;
+       Prog *p, *q, *q1;
        int32 autoffset, deltasp;
        int a, f, curframe, curbecome, maxbecome, pcsize;
        Prog *pmorestack;
@@ -667,8 +667,33 @@ dostkoff(void)
                                autoffset = 0;
 
                        q = P;
+                       q1 = P;
                        if(pmorestack != P)
                        if(!(p->from.scale & NOSPLIT)) {
+                               if(debug['K']) {
+                                       // 6l -K means check not only for stack
+                                       // overflow but stack underflow.
+                                       // On underflow, INT 3 (breakpoint).
+                                       // Underflow itself is rare but this also
+                                       // catches out-of-sync stack guard info.
+                                       p = appendp(p);
+                                       p->as = ACMPQ;
+                                       p->from.type = D_INDIR+D_R15;
+                                       p->from.offset = 8;
+                                       p->to.type = D_SP;
+                                       
+                                       p = appendp(p);
+                                       p->as = AJHI;
+                                       p->to.type = D_BRANCH;
+                                       p->to.offset = 4;
+                                       q1 = p;
+                                       
+                                       p = appendp(p);
+                                       p->as = AINT;
+                                       p->from.type = D_CONST;
+                                       p->from.offset = 3;
+                               }
+
                                if(autoffset < 4096) {  // do we need to call morestack
                                        if(autoffset <= 75) {
                                                // small stack
@@ -676,7 +701,10 @@ dostkoff(void)
                                                p->as = ACMPQ;
                                                p->from.type = D_SP;
                                                p->to.type = D_INDIR+D_R15;
-                                               
+                                               if(q1) {
+                                                       q1->pcond = p;
+                                                       q1 = P;
+                                               }
                                        } else {
                                                // large stack
                                                p = appendp(p);
@@ -684,6 +712,10 @@ dostkoff(void)
                                                p->from.type = D_INDIR+D_SP;
                                                p->from.offset = -(autoffset-75);
                                                p->to.type = D_AX;
+                                               if(q1) {
+                                                       q1->pcond = p;
+                                                       q1 = P;
+                                               }
        
                                                p = appendp(p);
                                                p->as = ACMPQ;
@@ -704,6 +736,10 @@ dostkoff(void)
                                p->from.type = D_CONST;
                                p->from.offset = 0;
                                p->to.type = D_AX;
+                               if(q1) {
+                                       q1->pcond = p;
+                                       q1 = P;
+                               }
 
                                /* 160 comes from 3 calls (3*8) 4 safes (4*8) and 104 guard */
                                if(autoffset+160 > 4096)