runtime: use uintptr instead of int32 for counting to next heap profile sample

Overflow of the comparison caused very large (>=1<<32) allocations to
sometimes not get sampled at all. Use uintptr so the comparison will
never overflow.

Fixes #33342

Tested on the example in 33342. I don't want to check a test in that
needs that much memory, however.

Change-Id: I51fe77a9117affed8094da93c0bc5f445ac2d3d3
Reviewed-on: https://go-review.googlesource.com/c/go/+/188017
Run-TryBot: Keith Randall <[email protected]>
TryBot-Result: Gobot Gobot <[email protected]>
Reviewed-by: Austin Clements <[email protected]>

Author: randall77

src/runtime/malloc.go

@@ -1075,8 +1075,8 @@ func mallocgc(size uintptr, typ *_type, needzero bool) unsafe.Pointer {
 	}
 
 	if rate := MemProfileRate; rate > 0 {
-		if rate != 1 && int32(size) < c.next_sample {
-			c.next_sample -= int32(size)
+		if rate != 1 && size < c.next_sample {
+			c.next_sample -= size
 		} else {
 			mp := acquirem()
 			profilealloc(mp, x, size)
@@ -1170,15 +1170,15 @@ func profilealloc(mp *m, x unsafe.Pointer, size uintptr) {
 // processes, the distance between two samples follows the exponential
 // distribution (exp(MemProfileRate)), so the best return value is a random
 // number taken from an exponential distribution whose mean is MemProfileRate.
-func nextSample() int32 {
+func nextSample() uintptr {
 	if GOOS == "plan9" {
 		// Plan 9 doesn't support floating point in note handler.
 		if g := getg(); g == g.m.gsignal {
 			return nextSampleNoFP()
 		}
 	}
 
-	return fastexprand(MemProfileRate)
+	return uintptr(fastexprand(MemProfileRate))
 }
 
 // fastexprand returns a random number from an exponential distribution with
@@ -1213,14 +1213,14 @@ func fastexprand(mean int) int32 {
 
 // nextSampleNoFP is similar to nextSample, but uses older,
 // simpler code to avoid floating point.
-func nextSampleNoFP() int32 {
+func nextSampleNoFP() uintptr {
 	// Set first allocation sample size.
 	rate := MemProfileRate
 	if rate > 0x3fffffff { // make 2*rate not overflow
 		rate = 0x3fffffff
 	}
 	if rate != 0 {
-		return int32(fastrand() % uint32(2*rate))
+		return uintptr(fastrand() % uint32(2*rate))
 	}
 	return 0
 }

src/runtime/mcache.go

@@ -19,7 +19,7 @@ import (
 type mcache struct {
 	// The following members are accessed on every malloc,
 	// so they are grouped here for better caching.
-	next_sample int32   // trigger heap sample after allocating this many bytes
+	next_sample uintptr // trigger heap sample after allocating this many bytes
 	local_scan  uintptr // bytes of scannable heap allocated
 
 	// Allocator cache for tiny objects w/o pointers.